Can’t delete NT user accounts

NT 4 sp6a Servers. There are two user accounts that I can’t delete. The error message is “The following error occured went trying to delete user “so and so”. The user does not belong to this group.” the title bar is User Manager for Domains. I’ve tried removing the user from Domain users and can’t. I’ve tried adding them to Domain users so I can delete but get the message they are already members. They do not show in the group Domain users but Domain users group shows in their group memberships. I can rename, change passwords, change from global to local. But I can’t seem to get rid of these accounts. I am on the PDC. Your help would be appreciated.
Thanks
Vicki