OK, I solved it, but not the way you proposed. This page Windows 10: only have built-in administrator account? - Windows 10 Forums introduced me to netplwiz. With that I was able to first create...

I have no way of doing a "clean install", since I don't have media and I don't want to lose all her personal data. The only thing I could do is to redo the upgrade, but having done it twice, I don't...

This sounds like a lot of work. Also I doubt she has the original (probably windows 7 or 8) product key -- we weren't asked for that when I did the upgrade and she is unlikely to have kept it from...

Nope, when I now go to Your email and accounts from settings, and click SIgn in with a Microsoft account instead, I just get a window flashing at me (I can barely see it happen, it goes by so fast)...

I am trying to help my neighbor upgrade to windows 10 from 8 (we have downgraded once because of these problems, and if I can't get this solved soon, I will have to do it again).

In 8 apparently,...