CCleaner is a freeware Software. It's an excellent tool which serves different purposes like system optimization, privacy and cleaning tool. What it does is that it removes all the unused files from your system making Windows to run much faster.
CCleaner also clears all the history pertaining to your online browsing. It also sports a full fledged registry cleaner.
So if you think of cleaning up your registry and speed it up the likes of which mostly only a freshly formatted and reinstalled machine would give you, this is the definitely the tool for you.