I think the first step is to make sure that you are using the latest version of Samba, which should support the authentication into the Active Directory domain. I think that you may also want to consider trying one more version of Linux, which does a very good job authenticating against Windows domains -- Xandros. I am not familiar with your actual error in the context of logging onto the domain, but I suspect this might be the case.
You mention the login screen. Is this the Linux main login screen, or is it somewhere after you logged in? The Linux graphical greeter will include a username and password prompt as well as the choice of session type. Usually KDE or GNOME are options or maybe they are the default, as is the case in distributions that fancy up the interface a bit. I am wondering: Are you trying to log in by authenticating against a Windows Active Domain Directory? You should first log in to the Linux PC and then authenticate for file and print sharing against your Windows primary domain controller (PDC).
If that is the case, once you are logged into the desktop, you should really look at this documentation at the Samba Web site: Chapter 9 of Samba User's Guide, which is called "Adding Unix/Linux servers and clients."
My only other thought is that the problem also might have something to do with Portable Authentication Modules (PAM). You can look for some debugging information in /var/log/messages. You might want to look in the /etc/security/pam_env.conf files for clues once you look at the messages.
Also you may want to post this question to the Samba mailing list, where they can inquire about additional troubleshooting data since what we have to go on here leaves a wide possibility of solutions.
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