I've had machines hang, or reboot out of the blue with no apparent cause.
The correct procedure to follow is Voodoo Repair, which involves dancing around the machine while swinging a freshly killed chicken by its neck. Of course, one should wear nothing but body paint and be suitably intoxicated.
On a more serious note:
- Check the fans and heatsinks. Remove any fluff.
- Reseat all cards. Pull them out until the edge connector is fully out of the socket, and then back in again. Gently.
- Reseat the memory modules as well.
In my experience, this will solve most problems - likely caused by intermittent contacts, thermal expansion or marginal overheating.
Should the problem persist, try running a memory test.
A "solid" hardware fault is unlikely as this would usually result in a BSOD.
Good luck.
/me resumes fight with a friend's machine which has the OS (xp of course) thoroughly f****d up by a variety of serious malware... Looks like FORMAT C: will be the only suitable remedy...