Apply Service Pack 1 - click here
Join the club - click here
Then go to Start > Control Panel > System > Advanced > "Startup and recovery" - Settings > Take the tick out of "Automatically Restart". Clcik ok next time this happens you'll get a blue screen note down exaclty what you see and post back back with the message.
Also try > Start > Control Panel > Admin Tools > Event viewer look at application and system logs for an error click the log and within the log will be a description of the problem and a possible fix.