Something else is wrong then. As there is no way possible to delete more space the you physically have on your hard drive though. Can you run a chkdsk on the drive and let it run when you reboot and see what it finds?
Right click the drive, select properties. Tools tab and select error checking. Check start now and select automatically fix errors. Click start and it should ask would you like to run it when you reboot, select yes.
also last time a did a disk check, it didn't find anything, and I had this issue back then too.