Okay, so I've looked through plenty of the "Computer starts up, but shuts off" but none of them seem to be the case. Here's my dilemma:
I'm upgrading my old PC piece by piece until totally overhauled.
Old Computer: Pentium D in mobo that came with it, 2 gigs ram, GeForce 7600 GT, 430 PSU.
I just upgraded my case to something with more heavy duty [more drive and card slots, 4 fans], power supply to 550, and video card to a 9600. Mobo has no video, so video card required. Here's what happened:
First time I tried to start it, I had missed the 4x connector from the PSU near the cpu [the extra power for the fan?] and though it turned on, the screen didn't, and it didn't seem like anything was actually running. Then, I plugged the 4x in, and it started, went through the POST, was approaching windows load, and just turned off. I pressed the power button again and it started again, but this time only went for a couple of seconds before shutting off again. Waited a few minutes, pressed it again, and it went through half of the POST before turning off. I didn't need to unplug or reset the power, pressing the button was enough to get it going again.
Left it off overnight, and in the morning it started like the first time. Almost to windows load, then off. Now, I get to testing. Switched video card back to 7600. Still died at POST. Switched power supply back to old one, with minimal hooked-up components... Dies shortly after POST. So I switched back to the new build. Same deal.
All I've deduced so far is that the new card was too powerful for the power supply, which was too cheap, and blew the mobo. Except that if the mobo's that bad, then it shouldn't even get as far as it does. I'm about to try resetting CMOS.

///rEI

Hi Rei
It sounds like a heat issue, in all the work you did perhaps you bumped the heatsink on the cpu. If it is not seated properly it will give you heat issues and trhe machine shuts down automatically after a certain temp. is reached.
DrD