My friend just built his computer a few weeks ago, had no problems for the first week and a half, but now for some reason his computer will restart w/o warning while he's playing any game. It doesn't happen while streaming videos, or doing any other task. He has already checked his BIOS to make sure his computer doesn't shutdown on blue screens, and he monitored his temperatures while he's in game and they all are normal. None of his stuff is over clocked either.

SPECS:
680W Power Supply
Q6600 @ 2.4
ASUS P5N32-E 680i nForce board
EVGA 8800gts
2 gigs of Corsair XMS2
10k RPM 150g Western Digital Raptor

What's the brand on the power supply?

Run memtest for at least 7 passes to stress-test the RAM (even though Corsair is usually problem-free).

Make sure all drivers are updated. Especially the video driver (since it crashes in-game). The latest Forceware driver is 169.21

Right-click My Computer and select Properties. Go to the "Advanced" tab and click "Settings" under "Startup and Recovery". Make sure "Automatically Restart" is NOT checked.

Good luck.

Not sure what brand, he didn't buy his power supply, the person who built it provided one, it says Sun-something on it.

another (more distant) possibility is a short somewhere, i've got a similar problem with laptop i'm having repaired, and the idea so far is that its a short in one of the fans. if graffick's ideas don't pan out, try setting the gfx fan to %100 for a few minutes and see if that makes the problem occur.

My experience on random starts while playing games are:

PSU
Video Card
Ram

Maybe he could borrow those items I mentioned and have a trial and error.