Open Start menu and type (search) for “Free up”, which should return the “Free up disk space by deleting unnecessary files” control panel item:
Start and let it scan your disk for files that might be deleted without harming the functionality of your system.
It will return a list like this:
If you also want to remove the windows.old and .bt folder you need to also scan for “Systemfiles” using the button that requires administrative rights.
It will return a list of potentially unneeded files and folders and lets you pick and choose which ones you want to keep or remove from your installation.
One of these should be “Previous Windows installation(s)” which has had a size of 17.7GB and eventually even 31.3GB on my disk as you can see here:
Also the “temporary Windows installation files” might be safely deleted and seem to correspondent with the $Windows.~BT folder:
Clicking ok, let’s you think about it again by asking the “are u sure” question:
It might not totally remove the Windows.old folder, but almost. (it left some files with a total of 1.4MB behind on my computer, which I moved to a TEMP folder to get them out of sight on the root of C till I have time to take over ownership and than be able to delete those leftovers, too.