This could well be that you have set the PC to reboot on error rather than reporting the error. Sometimes, on reboot, you also get the option to report the error to Microsoft, and even better, sometimes you get a report back telling you what's wrong. But that's not happening here, so you need to find out what's causing the crash. In Control Panel, select System, then go to the Advanced tab. Under Start-up and Recovery make sure that the 'Automatically Reboot' is disabled. You should then find that, the next time a problem occurs, you will get an error message rather than the machine rebooting itself.