Microsoft Office XP (all editions), unfortunately, seems to store some required information in *.bak files. This goes against the common convention that *.bak files are "backup files" that can be removed without any harm. As a result, I have added a "safety check" in CleanUp! 4.0 (and later) so that it will not delete any ".bak" files from the:

  ...\All Users\Application Data\Microsoft\...

and similar folders. In earlier versions of CleanUp!, this problem causes a minor inconvenience but does not cause any serious damage.