Stumbled upon something by accident. I was trying to remove c:\old.windows from my Windows 10 Professional system to get back about 18GB and none of the administrators command options seemed to work. By accident I brought up a CCleaner application and under the advanced section was a box unchecked called Old Windows Installations. I checked the box, ran the analyze program, and it deleted all of the Old Windows systems. Got back over 18Gb from the old system. Hope this helps someone else with a simple method to get rid of the Old Windows if you have no plans for restoring backwards.
All you have to do is run disk cleanup on C: then run clean system files, there you can remove windows old. For anyone who doesn't want to install CCleaner.
The Windows.old folder actually contains all the files and data from your previous Windows installation. It can be used to restore your system to the old version of Windows if you don’t like the new version. They will automatically be deleted in about 30 days or so.
And you can cheat like me and do endless clean installs without formatting and then from windows.old move my documents, videos, music, and downloads to my current install folders and delete windows.old from disk cleanup. That way I have a clean install with all of my old files and no wasted space, which fragments the drive, but a defrag fixes that.
Just run disk cleanup and one of the choices is to remove old Windows installations--it will remove the Windows.old folder and everything else associated with 7.