Clarification regarding windows 10 activation:
Windows 10 has a new form of activation called "digital entitlement". Basically, once you have a legally activated Windows 10 (either by upgrading free from 7/8.1, or pre-installed on pc), your computer is saved on microsoft activation servers as having "entitlement". Therefore, if you were to clean install windows 10 any time afterwards, you won't need to put a product key, because microsoft knows you deserve to have windows activated.