1 out of 5 wnt 4sp3 machines acts up

[mathog at seqaxp.bio.caltech.edu]

Ok, I'm stumped.  I've got what appear to be 5 identical WNT workstation 4
sp3 machines.  All have been configured to run off the primary domain
controller (Samba 1.9.19).  On one of these ONLY the domain logins
generates a single popup window which says: 

Incorrect password or unknown username for \\machinename\C$
and presents them with a username/password dialog box.

The users can blithely ignore this message, and nothing bad happens.
They are already logged on, and aside from this one warning, the
machine works fine for these users.  Or they can fill in the data and
make the box go away, but nothing changes when they do.

Any ideas what this might be?

Thanks,

David Mathog
mathog at seqaxp.bio.caltech.edu
Manager, sequence analysis facility, biology division, Caltech