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.
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