One of the biggest slow downs for computers is the crap that you accumulate while just browsing the web.
CCleaner combines a system cleaner that removes unused and temporary files from your system and also a fully featured registry cleaner.
I have been using CCleaner for 6 years or more and it’s the second thing I install after a new install of windows. The first being Firefox. Not only does CCleaner, clean up your temporary internet files, cookies, history, recently typed URLs, Index.dat files, last download location there is more but I am feeling a bit lazy…

CCleaner also comes with a registry scanner that will fix and or remove files that are no longer being used as well as removing left over application paths from removed programs.
It also comes with a program uninstaller, while I recommend using revo’s uninstaller this program is better than the standard windows uninstaller
CCleaner is a user friendly application that allows you to make a back up before you remove any files in case you are unsure about a certain file. This way if something does go wrong you can easily return your computer back to the state it was in before running the Cleaner. I have never had to do this however it is nice that it is there.