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#11
Yes, something is taking up that space. Do you have any unneeded programs on the computer.
What Anti Virus program are you using & have you done a full scan to check on any malware etc..
I did ask if you used Hibernate, if so disable it as it can use a lot of space & set how many System Restore points you have as this can also take up a lot of space.
You could also run System File Checker to see if there are other problems as well. This Forum tutorial explains how to do this.
SFC /SCANNOW Command - System File Checker - Windows 7 Help Forums
How long has the C Drive in service & have you ever reinstalled Windows at all. It might have a lot of old files on it that are not showing up in Tree Size & you may have to consider a reformat & reinstalling Windows again.
You do list a Backup Drive. What is it used for?. Do you have any Imaging software on the C Drive?.