If you have ruled out a virus or spyware, try shutting your running processes down one by one until you identify the culprit. This has happened to me twice in the past few weeks. The first was caused by the Official Big Brother website which I was frequenting and left open as a window. It was fine for 15 minutes or so and then the CPU went crazy. Once I closed it my cpu useage plummetted. On Saturday it happened again but this time it was Norton's playing up. It wasn't even doing a virus scan! I closed it down and started it up again and all was ok again.