CCleaner,
Superantispyware,
Malwarebytes,
WISECare365, and a couple different defraggers (Defraggler and UltraDefraggler is what I use) once or twice a week keeps me running like greased lightning. I also run
PeerBlock when I feel the need (starting to run is non stop in the background now a days). Also,
StopZilla is wroth buying, it is a VERY good catcher.
Pay close attention to what gets downloaded and where. READ EVERYTHING when downloading. Do not create shortcuts if you dont need to. Watch out for places like CNet, they attach a lot of crap to your downloads. Just generally cut down on the number of process that you have to run. I'm running Windows 7 Ultimate with 40 processes ( not on the internet or anyhting, just idle) and I can get it down further with more time.
Back up your data, and start fresh.
Just to be clear, save your important data ONLY. Go through and only take out what you need (personal mic., job information, tax stuff...ext). If you have some sort of bug, you dont want to save it and transfer it along with your saved data to your newly cleaned system.
Typically if you spend a day doing as everyone in this thread as suggested, you should ahve a pretty clean system in the end and you wont have to reload your OS. The cleaners/anti virus programs listed are very good as what they do, but if oyu still ahve a problem after that then you might want to think about a fresh start.