Help needed - PC shuts down immediately after startup

Hi-

A few days ago, my wife woke me up by asking me if I knew that my computer would not turn on. Thinking this strange, I got out of bed and went to investigate. Sure enough, it does not turn on. More accurately, when the power button is depressed, the computer will start up, fans start, etc, and then after 2-8 seconds, it shuts off.

I had left the computer on, running completely fine the night before. As far as I know, there was no thunderstorm or power failure that night. Somehow in the space of about seven hours, the computer shut off and developed this problem.

Thinking it was a power supply problem, I bought myself a new Corsair 650TX PSU. After installing that today, and giving the internals of the computer a good cleaning, the problem persists.

I haven't made any hardware or software changes in the last 4-6 weeks, during which time I used the computer heavily every day.

I don't see any places the motherboard could be shorting out. I did swap the reset and power button pins to see if the problem was a faulty power switch and the problem persists.

Any ideas on what to check next? Is my MB likely the problem? I don't get far enough into startup for anything to appear on my monitor so I cannot tell how far into the boot routine it gets before shutting down, though it appears to be not far at all. There are no beeps. Any help at all is greatly appreciated!

Specs:
CPU: Intel Core 2 Duo Processor E6400 with stock cooler
GPU: EVGA nVidia 8800GT 512 MB
MB: ECS 965T-A P965
PSU: Corsair 650TX
RAM: 4 sticks of AOpen 1GB (4 GB total)
Case: Antec P180

Also have two SATA harddrives and an ATA DVD drive, a floppy drive and a firewire expansion card.

 

Have you tried removing and replacing the CMOS battery? Is there any little green indicator light on the mobo when it is off? If not then buy a new watch battery and replace the CMOS battery.