For a quick and dirty listing, run a Command Prompt and type (for the C: drive, change for other drives) : DIR C:\ /S /A > files.txt

This will produce a (large) file called files.txt which can be viewed in a text editor or word processor and will list what files exist in which folders, with total at the end, but I don't recall for sure if there's anything hidden in XP that this won't reveal.

And here's another freeware reporting program:

http://www.snapfiles.com/get/foldersreport.html