Are you maybe running some software in the background, which could be the problem?
Do you get a bluescreen? Or does the PC just reboot?
If you get a bluescreen, what does it say? Maybe some error in a system file?
IF you dont get a bluescreen, maybe look in "Control Panel" - System - Advanced-tab.
Then go to "Startup and Recovery" and click on "Settings"
Then, if "automatically restart" is checked, uncheck it.
Now you maybe get a bluescreen with some info instead of just reboot. Or maybe not even a bluescreen.