CCleaner is one of my favourite utilities for Windows. It does several things very well.
One of them is removing the temporary files that various applications leave lying around. For example, the Windows temporary files directory gets filled up with various installers that have a tendency to never be removed as well as fragments of files left open during crashes. For another, Windows retains the option to let you uninstall every patch Microsoft issues. This may be useful in a mission-critical server situation, but on a regular machine it just wastes space.
I just had to clean up a very space-constrained virtual machine for work. CCleaner instantly freed up 1.4GB of space simply by removing crud left around by installers and other applications.
CCleaner also does a beautiful job of correcting Registry inconsistencies, finding dead Start Menu shortcuts, and various miscellaneous tasks like free space management and cleaning up the Add/Remove programs list.