>I just spent 6 plus hours upgrading a very fast machine from Windows 8 to Windows 10
>
>Terrible, terrible work on the part of the people who put that upgrade package together, but that's not the reason for this post.
>
>I have a folder with 90gb of stuff in it called windows.old.
>
>I can certainly use that space.
>
>Do I need that folder?

http://www.howtogeek.com/223821/what-is-the-windows.old-folder-and-how-do-you-delete-it/

Seems it's automatically deleted after a month.....