Power Supply Maybe?

My computer shut down without warning or error message the other night. Following the shut down the power button the front of the case did nothing when pressed. I unplugged the power cord, waited a few seconds, plugged it back in and it just barely had windows running and it happened again. Same lack of button functionality after it and now it wont even get as far as windows without dying. I'm running a P4 2.8 w/ 800 FSB, 1 gig ram, radeon 9600XT 256MB. Just got this machine a week or so ago, everything was working just fine. Ran BF2 and HL2 with no problems, all of a sudden this. I think its the power supply but I dont know for sure. Any and all help is greatly appreciated.

Also, I'm pretty sure its not heat related as I can see/feel all the fans running during time that it stays on, and before this whole issue arose it ran at less than 40C even running BF2.

Power supply issue seems quite likely here. Do you have access to a known good power supply (of sufficient wattage)? Could also be a problem with the thermal sensors on the motherboard - had a machine like that a while back, shutting down within 30 seconds even when turned on from cold - had to replace the motherboard in the end, as it was a thermistor on the board gone bad.

My buddy has a decent ps that i'm gonna try out today. I sure hope thats what it is. motherboard failure would be much worse. thanks.

I had this exact problem with one of my computer builds.  It ran fine for about a month, the started to shut down. In the end I had to get the motherboard and the power supply replaced.  My guess was that the initial problems came from the power supply. It had a relitivly weak +12 volt rail. The new one that I put in had a strong 34 amps on the +12 volt. Try the powersupply first, but make sure its a good one. Too bad mine needed the board also though.
My problem system:
A64 2800+, 512 ram, GeForce FX 5500