If you are upgrading or have previously upgraded to Windows 10 and the upgrade was activated by Microsoft you do not need an activation key.
If you have no product activation key, you can install from scratch (wiping the drive and re-installing) only if you have previously upgraded or installed Windows 10 and activated it.
When installing from scratch, at the beginning of installation it asks if you have a product key. Just click the correct answer. If you have no key, the windows installation will check for a previous activation on your computer and automatically re-activate it for you.
Did you perform an actual upgrade or a clean install? – Daniel B – 2016-04-10T10:04:25.313
1Both the answers given below are correct. – Don't Root here plz... – 2016-04-10T12:09:05.027