Bad Power Supply. Weak voltage power rail.
[There are three main voltages produced by the Dimension 4550's Power Supply.
1) 3.3 Volts
2) 5 Volts
3) 12 Volts ]
The Power On button LED functions as a diagnostic light also.
Dell Support > Dimension 4550 > Service Manual,http://support.dell.com/support/edocs/systems/dim4550/
Left-click on Solving Problems, then click on Power Problems
The Power Supply is just an ATX form factor version. The common Power Supply form factor used in MANY desktop computers.
(ATX form factor for a Power Supply refers to the size, and shape of the Power Supply's case.
Approximately 6 inches Wide, 5.5 inches Long, and 3-1/2 inches Tall)
HOWEVER, (Not shouting), there is an area of concern, should you try a Power Supply you may have available for a test, or purchase one.
It involves the wiring of the 20-pin ATX main power cable. More specifically the power cable's connector.
To explain view the standard wiring, set for a 20-pin ATX main power cable's connector,http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20
Looking at the photo to the far right, observe the color code of the wires, going down into the socket holes of the connector.
Look at the Lock tab on the side of the connector. This will help you orient the color code of the wires, and their placement in the socket holes, to the chart below the photos.
The connector on the end of the 20-pin ATX main power cable, is a Male connector.
Where the wires go into the connector, is the Back of the connector.
In the middle photo you can see the Front of the connector.
The wires going down into the ATX main power cable's connector, end in a female metal terminal.
The connector on the motherboard, (Shown in the photo to the Left), has Pins.
The metal female terminals go over the Pins.
The pins are numbered. The color code of the wires match up to the pins.
This is shown in the chart.
Now look at Dell Support > Dimension 4550 > Technical Overview > DC Power Connectors >
DC Power Connector P1http://support.dell.com/support/edocs/systems/dim4550/techov.htm#1101897
The illustration may not be accurate. You'll have to compare since you have the computer, and Power Supply in front of you.
For a time period Dell was a Proprietary manufacturer.
Meaning they wanted THEIR parts used on THEIR computers, and ONLY THEIR parts.
They had the motherboard manufacturers make motherboards according to their specs.
One of the specifications was that the pins would be CHANGED around, in the motherboard connector for the ATX main power cable.
They also had Power Supply manufacturers make the ATX power cable to their specifications.
This means if a user bought an aftermarket Power Supply, the color code of the wires in the ATX main power cable would Not match.
Many users did buy an aftermarket Power Supply, and installed it.
Burnt motherboards, fried Processors, and graphics cards, resulted.
Orange wires are 3.3 Volts
Red wires are 5 Volts
Yellow wires are 12 Volts.
Black wires are Ground wires.
Guess what happens when you plug a 12 Volt wire into where a 3.3 Volt, or 5 Volt, or Ground wire is supposed to Be?
My suggestion would be to look for a Power Supply that is an ATX form factor, is KNOWN to be good, and is compatible, for a test unit.
You, or someone you know may have upgraded to a better computer, and you may be able to borrow the Power Supply for a test unit.
Has the Correct power cables.
1) 20-pin ATX main power cable, OR a 20/24-pin ATX main power cable.
(20-pin connector that has an additional connector with 4 pins. Just use the 20-pin)
2) 4-pin ATX +12 Volt power cable. (Also referred to as P4 MB, and P4 ATX 12V)http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4
(Power for the Processor)
3) Enough standard 4-pin Peripheral power cables.
(Misnomered as Molex)http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral
(Power for the IDE Harddrive, and Optical drive/s)
You can change the power wires around in the ATX main power cable's connector.
Change them around, in an aftermarket Power Supply's ATX main power cable's connector.
Want to know how post in a Comment.