Update your virus definitions in your firewall to today, and run an anti-virus check after doing so... this will eliminate the possibility that it is someone else turning off your computer using a Trojan Horse on your PC...
I had a similiar problem last year and it drove me crazy! I eventually found out it was related to a bad stick of RAM. I removed the offending RAM and it worked like a charm after that. Good Luck, I know first hand how frustrating that problem is! Cohiba