CCleaner

High quality freeware program that lets you do a good cleaning of your system (temporary files, recent documents, Recycle Bin, Explorer and Firefox cache, registry, list of recent transactions of many programs).

Compared with Dustbuster can remove more things, but with more user intervention (Dustbuster is entirely automatic) For example, the registry cleaner in some cases may bring more harm than good (and not only with CCleaner...).

CCleaner is really good for privacy, and part of its program is devoted to this: You can erase the traces of the last operations of many programs.

CCleaner