You're probably experiencing a bluescreen. While you have a monitor etc. connected, could you please log in to your server, then:
- Right click My Computer.
- Click on Properties.
- Select the Advanced tab.
- Click on Settings under Startup and Recovery.
- If they aren't already set this way, uncheck Automatically restart under System failure, and check Write an event to the event log.
- Click OK, then Apply.
Then reboot your server and wait for it to crash. Let us know what the stop code etc. is.