I discovered that my desktop had a folder Windows AND a second folder Windows.old, both folders being 9 gigs. I started out working on 5 megabyte DEC RK05 disks on PDP-11s. I still think in terms of conserving disk space. 9 Gigs seems like a lot of disk space.
Anyhow Windows.old pops up when the machine gets upgraded, in this case upgraded from Win 8 to Win 10. The upgrade routine simply renames the old version of windows to windows.old and then puts the new version of windows into a new windows folder. This allows you to go back to the previous version of windows should you want to. In my case Win 10 has been working just fine, all my programs run properly, and I would never want to go back to Win 8, it was flaky. So, I decided to recover 9 Gigs by zapping windows.old.
You cannot just delete windows.old from Explorer. It is big enough to choke a hog, and it still carries protections against deletion that it enjoyed when it was the active Windows. Not a problem. Windows can fix it all. In Explorer find the master folder C:. Right click on the C: folder and select Properties. In Properties select "Disk Cleanup". In Disk Cleanup select "Clean up System Files". This will display a number of different files you can clean. Select the "previous versions" and let her rip. It takes a while but I gained 9 Gig of free disk space.