Could be one created by your Windows login (I think that's how it works) if you're not using an Outlook/Hotmail/live.com address for Windows login?

You could add an alias if you still wanted to use the account?

ETA: Info here - https://support.microsoft.com/ then click "Add an alias to your account"  

go to https://account.live.com

log in with your details

go to "your info"

click on "Manage your sign-in email or phone number"

from here you should be able to add a new email (add email).
once made you can set it as primary.  the existing one will just become an alias meaning you will still receive email to that address.

This is what I did when I changed mine from me@hotmail.com to me@outlook.com.

keep in mind this will affect any apps etc that needs to authenticate.  eg you will need to update email on your phone or outlook on your computer.

if you are using your existing email address as the username to log into your computer then I am not sure what this may mean for you.
it may mean you need to login with your new created email or something else could happen (I don't really know). 
if you are using your email as your computer login then I would create a local admin account just in case.  at least then you can get to any docs stored on the hard drive if something goes wrong.  

Someone may have more knowledge on this regarding what happens when you change your login/email name.

I only use local account for logging in. As per your advice I have created an additional email (alias) with a more sensible name.

And made it primary.

Cross fingers nothing breaks!

Thanks. :)

kiwifidget:

 

I only use local account for logging in. As per your advice I have created an additional email (alias) with a more sensible name.

 

And made it primary.

 

Cross fingers nothing breaks!

 

Thanks. :)

 

cool - so if you have outlook installed on your pc it might come up with a box saying the username or password are invalid. 
It will just need to be updated using the new email address.  It might not happen straight away and can take a few days or even weeks for this to happen.