Just went through this with my PC. There is a new virus Mc Afee has just updated a fix. Causes random rebooting. At the time I got it McAfee didn't find it so I had to reformat and start over. If your system has been fine and you haven't added anything new software or hardware then this may be your problem. Start your system in safe mode and you should be able to backup files and then reformat. Hope this helps....