Hint:
When you install 8.1 it will try to insist that you sign on with an e-mail account. Just put a dummy, but valid, e-mail address in with any old password and continue. It will then say something like "there is an error with that e-mail address" and asks if you want to continue without using one - i.e. using a local account.
I now remember having to do it with the pre-release, but it stumped me for a couple of minutes just now.