If my computer slows down I give it check up..
1. Run the antivirus
2. Run something like Adaware
3. Defrag the drives
4. As a back up to the antivirus as they don't all pickup everything I use something like HouseCall, it doesn't seem to upset my installed antivirus Avast!
5. Run CC Cleaner or similar aka as crap cleaner.. clears things out though you may need to redo your cookie set passwords. -
CCleaner.com
People sometimes have their own formula and some will likely say you don't need to do all those things but that one generally works for me and after it the computer runs much better.