Only formatting can really achieve a very thorough cleanup of all redundant files, orphaned files and other garbage, but of coure it also requires that you re-install Windows.
The next best method I use is a tool called "Ccleaner" which does a pretty good job:
http://www.the-pc-guru.com/Hyperlink...ork_in_ema.php
The only component of Ccleaner I would advise you not to use is the Registry Cleaner. Like most Registry Cleaners there is a risk it will remove a Registry entry which is required for Windows to run properly.