I found the reason for my trouble. I had to right-click SSMS and run it as Administrator. UAC is really a bitch.

Lowell (2/27/2012)

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adding the login is not enough...did you add them to the server role as well;

something like this is what you are after, i think:

--Add/Restore Restore Builtin\Administrators

EXEC sp_grantlogin 'BUILTIN\Administrators'

EXEC...