It’s generally acknowledged that Windows slows down with time: as you install and uninstall programs, crap gets left behind on the hard drive and in the registry that can make your computer act like molasses on a cold day. In the old days, pros recommended that every few months you simply reinstall Windows to solve this issue, but that’s using a pretty blunt axe. You can use a scalpel instead: CCleaner.
CCleaner will clean up files you don’t need, invalid registry entries, and the like. Since it does mess around with the registry and important system files, you need to be careful when using it, but overall it’s safe and does a good job of deleting unnecessary things without hosing your computer. If you’ve had your computer for a while and it’s bogging down, give CCleaner a try.