I coming up blank. The only reason the PC would restart itself is if it's overheating. If it was the PSU it would just shut off without logging out of windows first. So i don't know what it is.
this is not true... any exception error will cause the pc to reboot...If you have windows set to auto reboot in the event there is an error / BSOD etc.
To stop the auto reboot get into system properties from control panel. Once in system properties choose advanced tab, click start up and recovery "settings" tab.
Startup / Recover box appears, uncheck Auto Restart. The next time it BSOD's write the error down and google it.