Originally Posted by Ben
The next time it restarts, and you get the menu and the prompt to continue booting normally, see if there is an option that says "Disable Automatic Restart On System Failure", or something similar. If you boot with this option, then the next time Vista "resets", you should see a Blue Screen Of Death (BSOD). The BSOD will provide a particular "STOP" Error code, which will help in identifying the problem.
You can also go into your PC BIOS right after such a reset and then check your system health. Random resets such as the one you descriobe can also be caused by overheating. If this is the case, then you'll need to have a qualified service technician change your cooling system, unless you feel up to the task yourself.
Check that you have the latest drivers
If you are running Anti-Virus software, do a full update and system scan.
If you have over-clocked your system, which a BAD Idea to begin with, then use the default clock/voltage settings for your motherboard and see if that makes a difference.