Reboots can be caused by a number of reasons viruses are one,
corrupted windows files
faulty PSU voltages
are some more.
Details of error messages may help to pin it down.
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.
1. Go to Start -> Control Panel -> System (Windows+Pause works, too)
2. Go to Advanced
3. Under the Startup and Recovery section, click Settings...
4. Under System Failure un-check "Automatically restart"