A) When you hold a Power On button, (Switch actually), in for a count of 10 seconds, it turns the computer off.
(There is a Power On switch, located behind the plastic Power On button. Usually, the Power On switch snaps into the housing of the plastic Power On button.
Example of a typical ATX Power On switch,http://www.directron.com/atxswitch.html
The Power On switch turns the Power Supply on.
Old computers had a metal rod, that when the Power On button was depressed, the button pushed against the metal rod, which in turn pressed against a power switch located inside the Power Supply.
The design after that, was a power switch located in the Front Panel. (Plastic front of the computer) A large cable, (Wire) ran from the switch to the Power Supply.
Now there is the Soft Power On feature. (PS_ON)
The Power On switch is a small Momentary Contact Switch. Pressing it closes a circuit. The Soft Power On circuit.
When a power supply is plugged into power, there is a constant 5 Volts (DC) present.
This is the 5 Volt Standby.
Closing the Soft Power On circuit, redirects the 5 Volt Standby power to a circuit within the Power Supply.
This in turn 'excites' the Power Supply, and turns the Power Supply on.
Sorry to be windy, but I wanted you to know how the Soft Power On feature works )
eMachines are a budget computer. As such they save the consumer money.
However in order to do this, the manufacturer will install low quality parts, in some areas of the computer.
One such part is the Power Supply.
1) ALL the LED lights combined use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts
3) A typical Processor can use 51 to 125 Watts.
Depends on what Processor it is.
(Had you stated the Model Number I could have told you)
Enough power to spin fans, but not enough to spin fans, AND turn the Processor on.
Do you have access to another computer that is unused, but has a Known to be good, Compatible power supply?
It could be used for a test unit.
Tell me the Series, and Model Number of your eMachines, and I'll tell you what Wattage it should have, and what power cables, for the Power Supply.
(Some examples of Series, and Model Numbers, T1090, E4264, W2925, and so on. The T, E, and W are a few examples of the Series letter.
The numbers following the Series letter, is the Model Number)
If you wish I can direct you in removing the Power Supply, and installing the test unit. (And/Or a new Power Supply)