system crashes AFTER playing a game. Help diagnosing.

I have a friend who recently built a new PC, and he's experiencing
this odd issue where the system will lock up and reboot itself any
time he exits from playing any decently hardware intensive game, like
"Battlefield 2" or later.

He has an AMD Athalon 64 X2,
POWERCOLOR 26XT512M/D3HDMI Radeon HD 2600XT 512MB 128-bit GDDR3 PCI
Express x16,
GIGABYTE GA-M61P-S3 AM2 NVIDIA GeForce 6100 ATX AMD Motherboard
2GB or DDR2 RAM.
Every calculator I've used says he needs 350-420 watt PSU, he has a
520 watt.

I've watched his temps via the mobo software, Speedfan, and other
programs and they never climb very high. Even in the middle of a game,
after an hour, his CPU never gets above 50, his Northbridge sometimes
gets to 60 or 65 but usually 55-60, and his video card never gets
above 55 or 58C. So I don't think it's an over heating thing.

I've run memtest, drive tests (PartitionMagic, badblocks, checkdsk,
etc), video card hardware checkers, SiS Sandra, and everything seems
to say the hardware is fine.

We tried completely formatting and reinstalling Win XP Pro. tried the
default drivers, tried Windows update drivers, and tried Catalyst
drivers, and there's no change.

He never crashes IN the games and programs, just immediately upon
exiting them. No bluescreen, just reboots and auto-chkdsk's upon
bootup.
Any ideas what I should do or look into at this point?
Thanks for any advice!
-Liam