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#11
Here is the answer to the original problem, I found it somewhere else:
User Account - Hide - Vista Forums
Indeed, the “Run as administrator” command will not work properly when all administrator accounts are hidden–there is no box to type the password in. In Windows 7 (I have not tried for Vista) you are not necessarily locked out when you hid all administrator accounts.
Instead, select the program you want to run as administrator by holding Shift and right clicking it, then select the now revealed option “Run as a different user”. You can now manually type in the username and password of any account, including hidden administrator accounts. This way you can run the program as the desired administrator, even if the administrator account is hidden.
If you run the Registry Editor following this method, you can now unhide the hidden administrator account by deleting the key named after the administrator in HKEY_LOCAL_MACHINE\Software\Microsoft\Windows
NT\CurrentVersion\Winlogon\SpecialAccounts\UserList.
I tried it and it worked.
No need to reboot or do other complicated things...