OK, then check the directory c:\windows\instaler. If you see duplicate entries - e.g., same file name, same file size, but different date? Yes, you've been a victim.
The story we heard was that the Windows Installer (v2 or something) had a bug (or feature) where every update, such as to Office 2000, would place a duplicate instaler file in here because it thought it was the only instance of th install.
What does that mean? Well, you do an Office 2000 update 5 or 6 times because of patches or whatever, and all of a sudden you have 455MB files dropped in here because of a bug.
We found some systems with 6GB of wasted space due to these 455MB files duplicated several times for no reason except for an installer bug.
The quick fix? Open the directory (c:\windows\installer) and scan for duplicate files. To see which one's you can safely delete, run "regedit" and do a search in the registry for the file in question. No match? Delete it. Found match? Keep it. The typical behavior is that you should find 1 match from the group of files, and can safely delete the rest.