Hide account from login screen but can be used in UAC

The original question was different, but I have actually provided an answer to this question before. See my post in the question "Always display the last / default user Windows 7 welcome screen" - it explains how to set up UAC and hidden accounts together so you can authenticate to the account via UAC but not log into it from the welcome screen.

Essentially, what you need to do is configure UAC to prompt for the user to enter both their username and password manually, allowing you to enter a username which is hidden from the login screen but still enabled. Instructions on how to do this are on the linked post.

In the meantime, it should still be possible for you to recover the account without doing a full reinstall. Open a command prompt (cmd.exe), and type runas /user:USERNAME regedit.exe, where USERNAME is the name of the administrative account. It will prompt you for that user's password; enter it and hit enter. This will open the registry editor running as that user, from which you can go in and revert the changes you made, adding back the account to the login screen.

Re-read your question and noticed that you said run-as won't work. What happens when you try booting into safe mode? I seem to recall that with Windows 7 Home Premium, the built-in administrator account is disabled by default, but is enabled in safe mode, allowing you to log into it and make changes. Unless you have hidden the built-in account in addition to whatever account you set up, this may work.

Finally, if all else fails, you could try running a system restore from the recovery console. Boot onto your Windows 7 installation disk, and choose to "repair a windows installation" instead of install. You can select your installation, then will eventually get to a screen where you can choose to do a restore. This should change the registry back to how it was before you made the change, if it has an old enough restore point. Your files will not be modified by a system restore, although some system settings might be.

(as the question has changed, so must my answer.)

My idea is that one does not need to unhide the administrator account in order to use it. This account can stay hidden from the Welcome screen through the mechanism of Winlogon\SpecialAccounts\UserList.

This is based on the fact that one can always issue in a Command Prompt (or Run box) any command as administrator by using RunAs :

RunAs /user:admin "control userpasswords2"
RunAs /user:admin regedit

You can also in this way import a pre-prepared .reg file that will unhide the administrator account for your own use, then use another .reg file to hide it again when you finish with this computer.

RunAs /user:admin regedit c:\secret\directory\unhide_admin.reg
RunAs /user:admin regedit c:\secret\directory\hide_admin.reg

This way you can automate the hiding and unhiding of the administrator account.