I had a problem appear recently on my PC. Initially I noticed it was taking a long time for my PC to shut down. Then I ran all the scans, AV, Adaware and Spybot - several things were picked up, including a Trojan I got rid of all those, but still it was taking a long time to shut down, and there was a certain sluggishness overall, with processor usage being over the top in my opinion.
What I did was reinstall IE7 (it didn't lose any of my settings, I just ran the .exe file again), and lo and behold, I was back to normal (well, OK, maybe I wasn't, nor ever will be, but my PC was! ).
Might be worth a try Shirley?