I think what VoidMain was getting at was that you had said you were trying to change the password for the user ID "admin" whereas the default 'root' account for Windows NT/2000 is called "Administrator", so this could be your problem.
However... most _competent_ (non-MCSE?) network administrators will have changed their domain's administrator account to another name which looks like any other normal account in the domain, and may also have created a dummy "Administrator" account with only guest priveliges - so if you're trying to change the password on a system that was previously run by someone else, take that into account.
You are trying to change the admin password on a system to which you have the proper right to do so, aren't you?