It would be nice if CCleaner did what this tool does:
http://support.microsoft.com/kb/290301
Basically, if you have MSI's that fail to install the folder c:\windows\installer gets filled up with crap. I had 3 failed installs of VS 2005 SP1 and this resulted in 3 X 455MB crap MSI files in this directory. The cleaner should check the registry and find "orphaned" MSI's and delete them.