How old is the computer? Do you know the brand/model/size of your power supply? Have you ran a memory test?
First thing I would do, is replace the power supply if it's old (or upgrade it depending on how big it is/what type). Second, run a memory test. Most swear by memtest86, I swear it's like 50 years old http://oca.microsoft.com/en/windiag.asp
From Control Panel:
System > Advanced > Settings (under Startup and Recovery) > Under System Failure: Uncheck automatically restart
If reboots continue, go to the command line and run chkdsk /f and reboot, let it run its course.
Most common causes for unexpected reboots:
bad power supply
Though it can be any number of things. Bad sectors on the hard drive, corrupt system files, malware (closely related to corrupt system files), blend of software/drivers/frameworks that create OS errors/memory leaks, and the list goes on.
If you run through the above with no resolution, I recommend going to this link and follow the instructions to set dr watson as the default debugger and get nitty gritty with the crash dumps. http://blogs.technet.com/askperf/arc...ash-dumps.aspx
Good Luck, -dok