It is thoughtful of you Jack but don't worry about the points, besides my suggestions have not helped you as yet.
The only other thing I've been able to dig up is this:
from
http://support.apple.com/kb/TS1387
which says:
"These messages occur if permissions are incorrect on your designated music folder or on a folder inside your designated music folder. Permissions are settings that determine who can read, write to, or execute a file or folder on your computer. Every file and folder on your hard disk has an associated set of permissions.
Example: If permissions are correct on your Music folder, but incorrect on the U2 folder inside your Music folder, you would be able to add other music to your iTunes library, but not that new U2 album."
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I'm thinking this account which you say was standard and you upgraded it to admin privileges somehow has not worked on your iTunes folder.
Is your iTunes folder sitting in its default location, within your User / Music folder?
Did you drag that iTunes folder from another account maybe?