Essentially, it is necessary to take ownership of the Windows.old folder before you can delete it. It is currently owned by “System”.
Right-click on the “Windows.old” folder and click on “properties”.
Then click on the “Security” tab.
Then click “Change”
Type in “administrator” or your current user. Then click “OK”.
Checkmark “Replace owner on subcontainers and objects”
There is another checkbox for having all explicitly set permissions to inherit from the parent object. Checkmark that as well.
Now you can go back to the folder and permanently delete the Windows.old folder by highlighting it and pressing shift+del. This will fully remove the Windows.old folder from your system.