Hello
Maybe the problem comes from System Restore (creation of restore points or creation of a system image, which take space on the hard drive).
You can find help here:
http://windows.microsoft.com/en-us/windows/what-is-system-restore#1TC=windows-7
http://windows.microsoft.com/en-us/windows7/what-is-a-system-image
http://windows.microsoft.com/en-us/windows7/what-backup-settings-should-i-use-to-maximize-my-disk-space