This space is probably being used up by System Restore / Volume Shadow Copy. If you right-click on "Computer" and then select "Properties" from the produced menu, and then "System Protection". From the "System Protection" tab on the "System Properties" dialog box, select "Local Disk (C: (System)" from the "Protection Settings" list and then click on the "Configure..." button. From the "System Protection for Local Disk (C:", you should be able to see the current disk usage for system restore points and shadow copies.
You can delete the stored restore points & shadow copies by clicking on the "Delete" button, but be aware that this will delete all of your currently stored restore points.