Right-click 'My Computer' and select 'Properties', then go to the 'Advanced' tab. Click the 'Startup and Recovery' button to bring up the options box. Make sure that 'Automatically reboot' is un-checked. This will stop your computer from re-booting whenever Windows crashes, so you'll be able to read any error messages that are produced. That's a good starting point for tracking the problem down.
Regards
Nelviticus