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.
Once you have upgraded to Windows 10 and the system has been automatically activated, Microsoft’s product activation server will register said PC and consider it as a certified Windows 10 PC.
From there you’ll be able to create a bootable DVD or USB and perform as many clean installations as you want. Microsoft’s server will detect the PC and automatically perform the validation.
We have marked it as a “legal” solution because it is not improbable for the pirate activation methods to keep on working. In fact, some users have been able to upgrade to Windows 10 from pirated Windows 7 or Windows 8.1 copies.
Bottom line, if you have a PC with a Windows 7 or 8.1 license, you’ll first have to do the upgrade to Windows 10 (you can do it following this guide).After that, you’ll be able to use the ISO on clean installations.