Was told there should be a switch on the motherboard to test whether the power supply is supplying power. Can't find anything of that sort.

Ummmmm,..........No

1) First check the surge protector the computer is plugged into.
I have had several surge protectors, that just THE receptacle that the computer was plugged into, was bad.

"Hmmm, surge protector power on LED is lit, monitor has power, printer has power, and so does the router?"
Using a table lamp to check with, THAT receptacle was bad, and the others are not.

2) You're sure you have power going to the Power Supply, the next test is to see whether the problem is a $5 Power On switch, or the Power Supply.

Power Supply Fuse? Doubtful, but the holder is accessible from the outside of the Power Supply's case, (Rear), if so.

If you do not readily see one accessible from the outside, you're done.

Why?

Because there are Electrolytic Capacitors inside the Power Supply.
Even after the Power Supply is unplugged from power, these 'Caps' can hold a charge for weeks, months, sometimes over a year.
Stored charge could be released to YOU!
Shock is Bad to FATAL.

[ Radial Aluminum Electrolytic Capacitors, to be more exact,

http://en.wikipedia.org/wiki/Switched-mode_power_supply

http://en.wikipedia.org/wiki/File:ATX_power_supply_interior-1000px_transparent.png

In this top view the blue Electrolytic Capacitors marked with the letter B, are used in the Input Stage. These 'caps' are capable of storing Up To 100 Volts!
Can put X's in your eyes.

A tech Properly discharges them before working on the unit.
Most of the time the result is not worth the expense, or labor, in consideration of the easy availability of a Power Supply.

For a non-tech IMHO the only option is to replace.

The Asus P5G41-M LE motherboard uses a 24-pin ATX main power cable,

http://www.asus.com/Motherboards/Intel_Socket_775/P5G41M_LE/

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24

The Power On switch is bypassed, to see if the problem is the Power On switch, or Power Supply.
The test does not directly involve the Power On switch.

In the last link above, view the photo all the way to the right.
The Green wire is the Soft Power On wire.
ALL the Black wires are Ground wires.

A jumper wire is used to briefly connect the green Soft Power On wire, to ANY black Ground wire.

The preferred jumper wire is a straightened out paperclip, then bent into a U-shape.
The top of the U is wrapped a few times with black plastic electrical tape. The taped area is for your fingers, and thumb to hold onto.

U-shape turned upside down the 'legs' are used.
They go down into the socket holes, at the Back of the 24-pin ATX main power cable's connector.
The back is where the wires go into the connector.

One leg goes down into the socket hole with the Green wire in it, RIGHT NEXT TO the wire's insulation, and touches a metal terminal at the bottom of the green wire.

At the bottom of every wire going into the main power cable's connector, is a metal terminal. The tips are shown in the center photo.
This is what those female metal connectors look like, not installed,

http://www.molex.com/molex/products/datasheet.jsp?part=active/0002081202_CRIMP_TERMINALS.xml&channel=Products&Lang=en-US

The right side is the tip you see in the center photo.
The left side is crimped around the insulation of the wire. This is what the leg of the jumper wire must touch.

The other leg of the jumper wire goes down into ANY socket hole, that has a Black wire in it.

Power Supply plugged into power, power cable plugged into motherboard as shown in the right photo, the connection made with the jumper wire is no more than 2 seconds.
The Power On switch is a Momentary Contact Switch.

Power Supply comes on? You have a bad Power On switch,

http://www.directron.com/atxswitch.html

Power Supply does Not come on you have a bad Power Supply.

For additional questions please post in a Comment.
Regards,
joecoolvette

[Note*
The dangerous voltages are contained inside the case of the Power Supply. The Power Supply is a converter. It converts the AC electricity from your home, or business, into 3 Low DC voltages;

3.3 Volts (DC)
5 Volts (DC)
12 Volts DC.

The Soft Power On circuit uses 5 Volts DC.
In comparison two D cell flashlight batteries store 3 Volts DC ]

When your power supply died it probably killed the motherboard too. You can test the motherboard's viaablity with a stand alone video card.

Get a power supply tester and test your power supply. They are very easy to use. An inexpensive PSU tester that I can recommend to you is here:
http://www.newegg.com/Product/Product.aspx?Item=N82E16899705003&SortField=0&SummaryType=0&PageSize=10&SelectedRating=-1&VideoOnlyMark=False&IsFeedbackTab=true#scrollFullInfo

I'm going to base this solution on two scenarios from the info you have posted.

1) The power to your home/business went out. Now it's back on.

2) The power has been on all along, and the computer quit.


Based on 1) above;

The Power On switch could be bad, or the Power Supply.

A voltage spike could have occurred, and taken out the ATX power on switch.
(A generic ATX Power On switch is around $5. Example,

http://www.axiontech.com/prdt.php?item=78201 )

A voltage spike could have taken out a power supply that is of low quality, and/or on it's last leg.

The power supply used in an HP Pavilion a1100y desktop computer is of low quality.
Low quality components are used inside.
(Bestec, and Delta are two of the low quality power supply's used by HP in this time frame)

Plus add to the fact that the computer has some age on it, (5 years old?), and could be dirty inside.
Dirt,dust, etc. 'kills' computers and power supply's.

There is a simple test to perform to see whether the problem is the Power Supply, or the Power On switch.
(The Power On switch is located within the plastic Power On button assembly)
If you wish to know how to perform this test please post in a Comment.
(Free)


Based on 2) above;
The Power Supply is bad.
The test would be either to use a multimeter, and check the three main voltages coming out of the Power Supply,
Or,
Substitute the power supply with a KNOWN to be good compatible unit.
(250 to 300 Watt)

(Sometimes access to another known to be good, compatible power supply may be obtained from a computer that is no longer in use. The user has upgraded to a better computer.
Compatible being that the Power Supply has the required Wattage, and required power cables)

[The three main voltages produced by an SMPS (Switched-Mode Power Supply) used by the Pavilion a1100y desktop computer are;

A) 3.3 Volts DC (Orange wire)
B) 5 Volts DC (Red wire)
C) 12 Volts DC (Yellow wire)
In comparison two D cell flashlight batteries produce 3 Volts DC ]

This is HP Support>Pavilion a1100y desktop computer> Motherboard Specifications,

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00499325&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&site=null&lang=en&key=null&product=1118244

Looking at the motherboard illustration, and photo at the bottom of the page, we can see what power cables are needed for the motherboard;

A) 24-pin ATX main power cable
(Connector on motherboard illustration marked as ATXPWER1. In the photo it is whitish, and has two vertical columns of 12 square socket holes. It is to the Right of the Ram Memory slots)

B) 4-pin ATX main power cable.
(Connector on motherboard illustration is marked ATX12V. It is to the left of the Intel Northbridge chipset - Intel 915GV GMCH.
In the photo it is the whitish square connector with four square socket holes. It is to the Left of the
aluminum finned Heatsink, sitting on top of the Intel Northbridge chipset)

Other power cables needed are;
1) A SATA power cable for the SATA harddrive
2) One or two 4-pin Peripheral (Standard) power cables for the optical drive/s. (CD/DVD drives)

The Processor fan cable attaches to the motherboard. (CPU_FAN)
The computer case fan attaches to the motherboard. (SYS_FAN)

The ATX form factor is used primarily for describing a motherboard.
Form Factor denotes the size, and shape, and power cables for a Power Supply.

The ATX form factor for a Power Supply;
Size: Approximately 6 inches Wide, 5-1/2 inches Long, and 3-1/2 inches Tall.

A $40 ATX power supply with 300 Watts will do just fine. No need to buy an expensive Power Supply based on advertising as a direct substitute.

One example of an inexpensive reliable ATX power supply for a Pavilion a1100y desktop computer,

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1389575&CatId=1078

Has all the power cables you'll need. Actual maximum wattage rating is 350 Watts.
Information,

http://www.hardwaresecrets.com/article/332

For additional questions or to clarify anything I have stated, please post in a Comment.

Regards,
joecoolvette

eMachines uses low quality hardware components inside their computers. This is the way they can sell them at budget prices.

One main hardware component is the low quality power supply they use.
A generic power supply made by Bestec. (A generic power supply manufacturer)

(PSU. Power Supply Unit. The type of PSU used is a SMPS.
Switched-Mode Power Supply,

http://en.wikipedia.org/wiki/Switched-mode_power_supply )

Low quality;
A) Electrolytic Capacitors (Radial Aluminum Electrolytic Capacitor)
B) Rectifier Bridge
C) MOSFET/s
D) Wiring is too small in gauge
to name a few of the components.

However IMHO it is also best to check out the Power On switch.
There is a simple test to see whether the problem is a faulty Power On switch, or a faulty Power Supply.

The test involves bypassing the Power On switch. The Power On switch is not involved in this test.
A jumper wire is used on the Soft Power On wire, [ PS-ON ], and a Ground wire in the ATX main power cable connector.

If you bypass, and the Power Supply comes on you have a bad Power On switch.
If the Power Supply does Not come on you have a bad Power Supply.
If you would like to know more in detail on how to perform this simple test, simply post so in a Comment.

The Power Supply used is just a common ATX version.
ATX refers to the Form Factor of the power supply.
Regular size ATX form factor is approximately 6 inches Wide, 5-1/2 inches Long, and 3-1/2 inches Tall. (Height)

There are also differences in the pinout of the ATX main power cable.
Yours may have 20-pins, or 24-pins.
Power Supply's sold now take this into consideration.
Their ATX main power cable can be used as a 20-pin, or a 24-pin version.

[ General example of a 20-pin ATX main power cable, and it's connector,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20

Example of a 24-pin ATX main power cable, and it's connector,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24

Your motherboard may also require an additional power cable, commonly referred to as a P4 power cable, or P4 ATX 12V power cable, or just ATX 12V.
The actual name is 4-pin ATX +12 Volt power cable,

http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4

If you would like help in finding an inexpensive reliable Power Supply, please post in a Comment.
I will need to know what Model Number of eMachine you have. It's on the back next to the Windows product key, or up on the side of the computer tower.

If you need anything explained that I have stated here, please let me know in a Comment.
Regards,
joecoolvette

Make sure that the 20-pin ATX connector and the 4-pin aux. power connector are securely plugged into the motherboard and that the power supply's switch is in the "On" position. If the problem persists, you can try testing the power supply with a different motherboard and computer. If the power supply powers the other computer, it is likely the motheboard that is bad and not the power supply.
Hope this has been helpful. Good luck.

This is most commonly an issue of either the ACPI instruction being sent to an ATX power supply being misinterpreted as a restart command, or the controller in the power supply being faulty in general.

The root of the issue can be either your motherboard, or your power supply itself. To test this, try using a different power supply on the motherboard. If it works fine, try testing your power supply on another motherboard. If your power supply works fine there as well, then there's simply an incompatibility of some sort between the motherboard and PSU.

If on the other hand it doesn't work fine on the other motherboard as well, then you know the PSU is faulty.

The degree of fault in the PSU can be anything between harmless and severe; for safety's sake, I would recommend replacing the PSU entirely if a new/different PSU resolves the issue, as your PSU burning out may cause damage to your motherboard and/or (other) connected devices.

Either your power switch is not working properly, your motherboard is not sending the proper signal to the power supply, or your power supply is not coming on when it receives the signal from the motherboard. Most likely is failing power supply, but test your switch with a multi-meter, then test the signal coming to the power supply. You will have to look up the pin-out on your model, can't tell from the documents whether it is a 20 pin or 24 pin power supply connector to the motherboard.

the chances are that the power supply is not enough for the motherboard. or the main power connector has only 20 pins. the newer version of motherboards requier 24 pin power connector.

I couldn?t find much on the motherboard either. Over the years I have only seen a handful of boards with fuses so I doubt you have on your board. -Double check the new power supply and ensure it is designed for your motherboard. -Double check the power supply connections to the motherboard, the connectors should mate up exactly to the motherboard. -If you did remove anything double check for shorts, grounding and so on. Any of those can cause the same symptoms. -Test the case power switch, it could be bad. -Some power supplies have on/off rocker switches on them (in the rear by the A/C power socket), make sure it?s on. -Most power supplies have a 115/225 volt switch in the rear by the A/C power socket; make sure it is set to whatever is appropriate for your country. I assume you use 115V like we do down here. -Make sure the A/C power cord is working. -Test the power supply in another computer or if not available you can generally pickup an inexpensive tester for $10 to $15 at a computer store that sells parts. -If you get this far and still have no solution then my guess would be the old power supply had a power surge on its way out and damaged something major on the motherboard. You can look on the motherboard for obvious signs of damage; however, unless you know what you are doing I don?t recommend the typical user try to perform repairs of this sort. Let us know how things go or if we can help further.