Debian Sarge version of Samba.  When trying to connect to the server (either
\\server\ or \\server\share\), Windows Vista will pop up the dialog box asking
for credentials after a short wait, and the following will be logged in the
Samba log.

[2005/08/06 10:12:24, 1] smbd/sesssetup.c:reply_sesssetup_and_X_spnego(593)
  Unknown packet in reply_sesssetup_and_X_spnego
Relevant config:

[global]
        map to guest = Bad User
        obey pam restrictions = Yes
        passdb backend = tdbsam, guest
        syslog = 0
        log file = /var/log/samba/log.%m
        os level = 100

[dump]
        comment = File share
        path = /dump
        force user = dump
        force group = dump
        read only = No
        create mask = 0777
        directory mask = 0777
        guest ok = yes