Fixed it - I am soooo silly -- how could I have not seen it!!! I set a program called "Folder Lock" to lock away 13.xGB of stuff, now when I unlock it. I get the full size but when it is locked it hides the used space.
http://img278.imageshack.us/img278/8272/untitledks4.jpg
Maybe a couple of gig off, but now I've solved the mystery I can put away that Windows XP disc.

Thanks to you guys who came up with some good answers, thanks.