I have exactly the same problem with just one user.
Just got a batch of new PC's a couple of them are running XP.
I can add every domain user to the local admin group except for this one individual.
I even deleted her user and recreated it, but still had the same issue.
Its definitely related to the one user as I get the same issue trying to add the user to another local admin group
In the end I just went with a local profile and mapped drives.