Random restarts on new build

I have been building systems for a bit now, but i have NEVER encountered this problem: the system works great for 1 hour to 5 days; then occasionally and without warning, the monitor goes off and the system restarts. Usually it will come back up after hitting the reset button. This is the wifes work computer and is not shut off for the night, after use, etc.

I have these components: M/B-P5VD2-MX SE, SB Audigy 2, NVidia 5700 ultra video card, 1gig of corsair 400mhz ram, 480 watt 'locally purchased from a computer shop PS and an antec piano black case.

I have reformated the hd, reloaded the OS several times, taken the cards and memory out to test for hardware issues. The first time this happened, I did find the video card not sitting all the way in the slot due to it hitting one of the memory slot clips. I was able to quicky fix that problem, but the problem continues.

Any help will be greatly appreciated!

Thanks in advance!