I had this problem yesterday too :S, you may notice (if you've eliminated virsus), that it's
mostly when you connect to the internet, what seems to solve it for me is double clicking the Internet Explorer icon when i first log on, browse the net for a couple of mins and it goes to 0%
cpu S.
Strange fix I know but

, I think it may be somthing to do with Windows updates but I dunno

.