this is more than likely software causing the problem.
have you updated any drivers lately or installed any new software?
try system restore to a time it was ok,or through process of elimination remove one by one you last installed program untill its ok.
have a look in control panel/administrative tools/event viewer for clues.
also untick the automatically restart box in control panel/system/advanced/start up and recovery.now you will get an error message instead of a restart.note it down.