You should be able to open an elevated command prompt (CMD.exe) window tho'.
You can add a user there.
optionally to put this user into the group of admins, type this after the first command
net user /add <username> <password>
the problem you encounter happens with the built-in "Administrator" account.
net localgroup administrators <username> /add
Or if you stop/disable app-dependent windows services.
Oh and by the way... if you want to make it simpler you can of course go to the user accounts via the control panel, instead of via "Settings" ;-)