Windows – Change user for automatic login

I'm curious how your account became a guest account. That sounds like uncharacteristic behavior.

First, try booting into Safe Mode by pressing F8 before the windows boot animation appears. By default, the built-in administrator account will be enabled when you boot into safe mode. Try logging into it, then you can reset the permissions on your original account from there, then reboot into normal mode.

In order to achieve what you originally wanted, all you had to do is to remove passwords from both user accounts (i.e. specify an empty password for each). That way, if you have more than one interactive user registered on the computer, Windows will boot to the welcome screen giving you a list of users and the opportunity to log into any of these accounts simply by clicking a "Sign In" button (no password to enter).

And there's no need to fiddle with netplwiz for that. The "Users must enter a username and password to use this computer" box should remain checked.

Whatever you are experiencing must be a consequence of some peculiar set of changes you made through netplwiz. I suggest you revert it to the way it was originally and simply specify an empty password for each account.