WE need to see if there is an error message or if its shutting down through overheat,
Overheat check all grills are clear and fans running OK, take cover off and see how dirty its is inside especially round the cpu heat-sink and fan.
Auto-reboot disable: When Running windows and it crashes you will get a blue screen and it will automatically restart, often it will restart too fast for you to see the error message. To disable auto restart on system failure, so you can see the error message.
- Go to Start -> Control Panel -> System (Windows+Pause works, too)
- Go to Advanced
- Under the Startup and Recovery section, click Settings...
- Under System Failure un-check "Automatically restart"