Cannot see users in Local Administrators group!

I've got a Win2k machine with SP4 on it, and it's doing the weirdest thing. I cannot enumerate anyone in the Administrators group. If I open a users' account and show it's membership, it shows as a member of Administrators. If I try and add a user that I know is already there, it won't allow me to add it. If I add a new user to the Administrators group, it allows the add, and I can see it in the Users' group membership. But I cannot see what users are a member of the Administrators Local group at all. I've tried adding it to a domain and removing it again, I've tried re-installing SP4, and other things. But nothing seems to work. Does anyone know where the Administrators group membership is stored? Or why I wouldn't be able to enumerate the local admins group membership?