I can confirm that - adding the user to the USERS group works fine.  I'm trying to find out what it is I need to do (unlock/allow) so that I can add the user to the GUESTS group and for it to work.

As mentioned SQL Server runs fine like this, but the Agent does not.

The reason I want to add it to the GUESTS group is so the user that's running SQL has NO permissions on the box, except for the file/registry permissions I specifically unlock so it can do what it needs to.

Only thing I have left to do is the Agent.  SQL is quite happy running under a user that is part of the GUESTS group.