Hi all, I'm new here, could use some help.

Christmas EVE while playing some Left 4 Dead my computer randomly SHUT off. I figured it overheated or something and let it sit for a while. When I went to turn it back on again the fans just spun for a second then it died again. It did this every time, so I removed the parts one by one to try and find a problem. Turns out when the video card is not seated it starts running just fine, even the fan on the card runs. But when it's back in it refuses to boot again.

I tried resetting the CMOS, but to my dismay it didn't help. I have no BACKUP card or anything to test, and no onboard video so I'm stuck in the dark here. I sent the video card back for RMA and had to wait 2 1/2 weeks due to Xmas and New Years. (great timing I know) So when I finally got it back I figured all would be RIGHT with the world again. But of course with my luck, it did the exact same false start thing. So I've got it narrowed down to the motherboard mainly, or possibly the video card still, or on a long shot the power supply, but this whole setup has worked fine for many months so I really don't know.

My specs are:
Windows XP
AMD Athlon x2 6000+
MSI NX8800gt OC
4 gigs of ram
Asus M2N-SLI mobo
500W psu

None of the parts there are more than 10 months old. Any help is really appreciated.Borrow a power supply of EQUAL or greater wattage and see if it boots.