Last night I left my PC to shutdown. Still on this morning. Then I found it was completely unusable. 100% CPU all the time, even after a reboot.
Checked the service packs installed and the latest was installed on the 18th - Medcia Centre update. Unistalled it and normal service is resumed.
I think I may go the full monty now and uninstall Windows altogether. Linux here we come. The only thing I use Windows for is playing games - about all its good for really but my current PC does not have the grunt to play anything from this millenium. So there's nothing left that I need Windows for.