I've just built a new pc and I'm getting random reboots too. my spec is
as follows...

Vista Ultimate 32bit
Intel Core 2 Duo 2.66GHz
Abit AB9 Pro Motherboard
OCZ 2GB (2 x 1GB) PC2-6400C4 Dual Channel Platinum
BFG GeForce 8800 GTS OC2 320MB
SATA 2 300GB Seagate Barracuda
Corsair HX 620W ATX2.2 PSU

My rig is water cooled and has fans, so I know it's not a temprature
thing. The 620W power supply was £100 and has very good reviews so I've
got more then enough juice. I've swapped the memory sticks with a
friends pc but still it reboots. I've tried different Nvidia drivers
including beta drivers, I've even tried swappig the video card with a
friend who has the same card. I've formatted it several times, tried
installing vista onto a spare 200gig IDE drive but still no joy, I even
flashed my motherboard bios which I think has made the random reboots
less frequent but can't really be too sure.
Being Vista the memory dumps are usless and don't give me any
explination to why it unexpectially shut down. So it could be the
motherboard or cpu? but I'm hoping it's just a vista issue and that the
upcomming release of service pack 1 will help.

Any ideas?