Windows 10: Reactivating Windows 10 after formatting pc but I can't log in to a Microsoft account

So I bought a used pc that was formatted before I bought it but now Windows isn't activated, I haven't got any product key and I haven't bought a license myself. What do I do???

:)

 

Windows 10 Tweaks

As you may already know, Windows 10 (as well as Windows 8) allow you to log in with a valid Microsoft account, instead of creating a local account that only works on your machine. However, you may prefer to keep a local account for privacy reasons. If you do, though, some Windows 10 features—like Cortana—may not work. Despite the default behavior, the Windows 10 Store is not one of them. You can still log in to your Microsoft account for just the Store without switching to a full Microsoft account. The process, however, is not terribly obvious. To do so, follow these steps:

• Launch the Store from your Start menu.
• Click the user icon next to the search box.
• Click “Sign-in” from the menu that appears.
• Choose “Microsoft account” and log in like normal.
• When the “Make it yours” box appears do not enter your password. Instead, click “Sign in to just this app instead.”

from Lifehacker

 

register PC with microsoft; use local account to log in

Just simply log in with your Microsoft Account.
Switch to Microsoft Account in Windows 10

Just go back to using a local account:
Switch to Local Account in Windows 10

Hopefully the tutorials are step by step enough.

 

Strange Windows 10 User Account issue.

Unfortunately I had done those steps numerous reboots, it just, reappears for no good reason. The base account isn't even a Microsoft account, it's a local account.

I haven't quite tried the built in administrator account on W10, but so far in my experience on my own computer, the built in account seemed to have absolutely no more control than I did on my own.

It's just strange, I go in, it warns me twice about deleting the account, then it's gone. Reboot, and it's back?

EDIT: Just to specify, both command line and Netplwiz both say only HIS account exists on the computer, it doesn't list this rogue account. This is what I meant in my first post about the fact that even Windows doesn't seem to believe it exists, HALF the time. *Frown