It is the only one and almost correct answer. Missing is the fact that before upgrading you have to be signed in to Microsoft account and logged in your PC with Microsoft account. NOPE- you dont have to have a Microsoft Account. I've done at least 9 reinstalls on customers computers after the original OS, either W7 or 8.1 was upgraded to W10 , and none had MS accounts. The OS still runs better on a fresh install compared to a upgrade. Fun jumping between 2-3 computers at once One of my workshop Computers has had 3 reinstalls so far -- no MS account on it. Hey- got to break W10 to fix it What you have to do is first upgrade what ever OS you have ( W7 or 8.1) Make sure its activated. ( IMPORTANT) You can then wipe the hard Drive clean, or change hard Drives ,do a fresh install of W10. When you upgrade to W10 the motherboard is registered on MS's servers. This means if you ever need to reinstall, no product key is required, as its the hardware thats registered, not the licence key. Microsoft are using generic keys, so if you do a install of W10 ( upgrade or fresh later on), use a key finder, do a google search and there will be lots of people with the same key. The "gotya" is if your motherboard dies you need to buy a new key/licence. Its enforcing the OEM licencing you can say. Dont know what will happen if a new PC dies after a few months and a call is made to MS, in the past they would give you a new code to enter. If a Computer has not been upgraded from a previous activated OS, then it cant activate. (unless you enter a new W10 Key) If you have a insider Program installed build, thats when you have to have to sign in to your MS account. BUT the "gotya" on that at the moment is Microsoft have blocked all previous active keys. If you need to do a reinsatll its currently showing as Product key blocked. As long as the original install build was activated and you dont reinstall it, it will still ( or should be ) active. This "problem" should be resolved once they release new builds, whenever that is.