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 sp_addsrvrolemember 'BUILTIN\Administrators','sysadmin'
after you've done that, make sure you are really part of the Builtin\Administrators group:
EXEC master..xp_logininfo
@acctname = 'Builtin\Administrators',
@option = 'members' -- show group members
does your windows login appear as part of that result set?
Lowell