If your computer powers on for 2 seconds and then shuts off...

....check the heat sink, especially if it's a Intel Socket 775 processor.

I once built a computer for a client, and made the mistake of building it for him at his house. I was young and always assumed that brand new parts would work. I assembled the computer tower, powered it on and...

A quick flash of the lights, a quick spin of the fans, and then darkness.

I ran through the normal pull-every-single-component-out-and-replace-them-one-by-one routine, but to no avail. The same thing would happen each time I pressed the power button; the computer would spring to life for 2 seconds, and then immediately turn off.

Here was the problem, and it is not a common one. I was using a 3rd party cooler/heatsink (this was meant to be a gaming rig), that didn't sit on the processor like traditional heat sinks do. Because of this, there was not enough pressure being applied to the processor itself. For the record, I had used the appropriate amount of thermal past.

The amount of pressure being applied is key in this situation. The BIOS of a Socket 775 motherboard needs to detect that the processor is under at least 17 lbs of pressure before it will boot (according to the Intel datasheet). 17 lbs of force is a normal amount of pressure between a processor and a heat sink. It ensures that heat is will be easily transferred from the processor and up through the heat sink. The "pressure check" by the BIOS is to prevent the CPU chip from melting down if it is not being properly cooled, but it can be a headache for someone building the computer since this "feature" is not well known, and they may not know to check for it.

I ended up buying a new motherboard to get the client up and running. A few months later, my brother pulled the original motherboard out of the closet, put a stock cooler on it, and ended up with quite a nice gaming machine!