I have recently seen a issues on SERVER( WINDOWS 2003) in which significant difference between the “Size on disk” for the root of a volume, and the reported amount of “Used space” in the volume properties. While considering this, I noticed that my own C: drive had a mysterious 20GB difference.
Above link was very much helpfull to troubleshoot the issue. However my problem had been solved after running "chkdsk" in read mode.
Special thanks to Dennis Middleton “The NTFS Doctor”