Patio, you need to be more observant, you've done this a few times.
Electric Phoenix this is indeed a very unusual problem. I've done this in my computer. C: properties say 62gig used, put right click properties on everything in C: is 53gig. I know that there is some difference between the way file size is reported (it's complicated about sector sizes if you really want an explantion I'll tell you) but I wouldn't have thought it would have been that bad. Do you have lots of small (few kb) files? They can cause this problem.
What is the size of shared documents and admins documents? Please take a look in your recyle bin, you never know. On your system properties, go to performance virtual memory, what is your paging file size?
Did you try using chkdsk? Try analysing your computer with defrag and take a screenshot of the graph.
Try deleted these files you've put on it, and seeing how the size looks then.
Aside from that all I can think to do is format, retry again and hope for the best.