If you have spware/crapware it will sometimes max out your processor.
Not to alarm you, but try a scan of ad-aware SE, xblock freeware etc... I usually do a couple different to find all the junk if it's badly "infected". Be aware that if you remove some spyware it will break the free software you might have that require it. Sometimes free isn't worth the hastle!
Lastly - trojans can also max out your pc - Mischel security's Trojan Hunter will run for free (30 day trial - iirc) and worth knowing about if you don't find anything else.
good luck!
sl33py