Random freezing in games / at desktop

I've recently built a new computer with: intel core 2 duo, 2x evga 8800gt's, asus p2n-32 sli premium (using it's onboard sound) and 4 gb corsair ram. Ever since I've installed vista x64 on this machine, I've had random lockups while playing games and even will idling at the desktop. It's done it while play COD4, BF 2142, crysis (even did it when I paused it for a while and was left on the pause screen) and like I said, at the desktop. Then only way I can restart it is a hard restart or just shutting it completely down. And when I say random, I mean random. I.E. I just started my computer up and loaded COD4. 5 mins into the game it froze. I then restarted and played it again for an hour with no problems. I noticed the time on my g15 keyboard was still ticking like nothing was wrong. And sometimes I'll leave my computer for a few hours comeback to it to be frozen at the desktop. Any ideas?