OK, I have been watching this for a while now and decided the following.
The computer is not "restarting". The screen goes blank, the monitor says No Signal, Then all the drive lights flash (like on restart) and everything goes back to normal. This all happens in less that 15 seconds. So it's not a reboot. Could it be the driver for the NVDIA video card? I updated once. Maybe roll it back? Other than that Win 7 running good on the machine now. I also recently changed the power supply to a Corsair 600W The power seems to drop briefly when all the drives are spinning. But I think that's to be expected. I can see it in the case lights. The machine is clean and the temps are great! Kool huh!
Raidmax ATX case
Asus K8V Deluxe MB
AMD Athlon 3400 proc
3 G ram
NVDIA GeForce 5900 graphics card
Soundblaster digital 5.1