Vista - Weird error message when changing password in Active Directory

When changing a user's password in Active Directory Users... I get a strange
error message (and the password change fails):

Windows can not complete the password change for the user <insert user name>
because:
The system can not find the file specified.

Also can not add users.

However: NO EVENTS are being generated associated with this message in any
of the event logs...

Anyone have any idea WHAT file is missing? I have searched Microsoft.com
from one end to the other and can't find this error message.

(This started when the disk filled up about a week ago...)

Thanks,

Ed Otto