Windows Vista My Computer shows 30Gig used space more than is there

I am working on a ThinkPad with Vista Business on which I have cleaned out all documents and user files. When I look in My Computer I see that the C drive shows only 20 gig of free space with 69 gig of total space.

If I total up the size of all folders and files remaining after I cleaned it out it only comes to 29 gig of used space. I have set all hidden files to be shown including OS files.

Where is the remaining 20 gig? What or where might I be not looking at here?

There is a service partition where the installation files are sitting but even that is only 5 gig

Thanks for any thoughts