How to activate Windows 10 from the ISO during a clean installation
If you have tried to activate Windows 10 from the clean ISO installation with your previous Windows 7 or Windows 8.1 license, you’ll have noticed that you’re not able to activate it. Don’t worry, it’s normal according to Microsoft.
Contrary of what we thought, a valid Windows 7 or Window 8.1 license won’t be able to activate Windows 10 because this uses its own key, hence the activation error.
Solution (legal)
A Windows 7 or Windows 8.1 license allows you to use Windows 10, although it doesn’t allow clean installation at the moment, and so the only solution so far is to upgrade your Windows.