Yesterday I've upgraded to Windows 10 Home 64 bit, from my Windows 7 64 bit Home Premium. Then I clean installed it, after the OS was activated.

I used a local account and I have never switched to any of my Microsoft's e-mail accounts. The installation went fine. So, right afterwards, I have customized the Privacy settings, from the Settings Menu. In short, I have disabled (turned to Off) almost everything found in there. And I have also set Cortana to not get to know me. She isn't available in my region, anyway. So all good.

Then, and just out of curiosity, I've tried to set up the Mail app. I've added two MS accounts. They could not synchronize, of course, so no problem. I was just curious.

The next day though, when I powered on my computer, one of those two accounts had become the Microsoft account for my computer!! So, I was asked for the password on the Lock Screen, right after booting up! Now this specific password for that account was created with a password manager, so it was a long alphanumeric string, which I could not remember, of course!

What saved me, from reinstalling the OS, was that I already had Windows 10 on another SSD. So I unplugged the disk with my "main OS", plugged in the other one, booted, powered on my external HDD, opened the file where I keep those passwords, wrote it down on a piece of paper, plugged back in the "main OS" SSD, entered the password and finally saw my desktop. Immediately, I've switched to my local account again.

- Why did Windows 10 arbitrarily and without any notification set my OS from a local account to an MS account?
- Can the Mail app provoke (cause) something like that by itself, and without notifying the user?!

I'll make sure this will NEVER happen again, but I'd appreciate your input.

Thank you.