Unable To Change Passwords

Bit stuck at the moment, wondering if someone could help.
Running a windows 2000 server with various 9x clients aswell as 2000 and xp clients.
When it asks them to change their password, it won’t allow them.and comes up with the error message: “unable to change the password for Microsoft Networking because of the following error: Access has been denied.”

it does this for 9x and nt clients

Has anybody come accross this before?

Chers