UNIQUE and Strange random restart problems....

So I got my computer back from the repair shop... I was under warranty, so I figured I'd let them do the work than pull my own hair out. However, this time, I feel as though I need to deal with my problem...

So everything boots fine; the welcome screen for XP shows up and suddenly the computer will flash to a blue screen and reboot. Restarts, loads into windows; so I disable the automatic restart option.

These restarts happen randomly; either right as I load into Windows, or as I am loading the system up. Usually, when Windows loads up fully it rarely, but still, reboots. The first message I got was a problem with savrt.sys... a Norton Anti-Virus file. So... I uninstalled Norton, and installed AVG instead. Rebooted 3 times, no problems...thought everything was fixed.

Not so fast. Suddenly I get an IRQL_NOT_LESS_OR_EQUAL error; Booted up Memtest86+; ran it 3 times, no errors.

Strangely enough, ever since I uninstalled Norton and installed AVG, whenever the restart occurs it's as Windows is loading and the AVG interface taskbar logo gets an exclamation point and the system restarts.

I know this is complex, but as I said, these are random restarts. I've checked the WHQL--drivers come back well. Memory is good. Graphics card isn't overheating and when the system boots I run Age of Conan very well. Nothing is overheating, all fans work. Could it just be a bad reformat by the computer company (iBuypower)? My other thought is PSU possibly, but that idea goes out the window since when Windows loads up I can run it fine under heavy load (Age of Conan on max for 2+ hrs)

Thoughts? BIOS? Bad Windows install? Solutions?

Thanks.

Computer Specs:
Intel Pentium D 840 3.2 GHz
ASUS P5ND2-SLi Deluxe
2GB DDR2667 Corsair
nVidia 8600GTS 512x2