I have a old windows 7 cd key but I don't know it gonna work on windows 10 or not.I read a forum back in 2016 but still don't know it gonna work in 2017 or not.
https://www.thurrott.com/windows/windows-10/75159/can-still-use-windows-78-x-product-key-activate-windows-10
https://www.thurrott.com/windows/windows-10/75159/can-still-use-windows-78-x-product-key-activate-windows-10