Well the only other option I can think of (other than a clean install) would be a
Repair Install which works as an
In-Place Upgrade and should move all non-essential files/folder to a new
Windows.old folder (including those that are in the current Windows.old folder). This will then allow you to run the
Disk Cleanup app to remove the windows.old folder.

Tip
You may want to try renaming the Windows.old folder to another name so that the contents of it are also moved to the new windows.old folder. This will prevent a second .old folder being created