You have set up a new Samba primary domain controller (PDC) like in Recipe 11.4, and you did not have a domain controller before, so your clients are not configured to log in to a domain. How do you login from a Windows NT4 client?
First, confirm that Windows networking is set up correctly: TPC/IP and Client For Microsoft Networks must be installed, and the appropriate network settings in place, which you'll find in Start → Settings → Network and Dial-up Connections.
Then, go to Control Panel → Network → Identification → Change. Select the Domain button and enter the domain name, which is the workgroup name in smb.conf.
Reboot and log in to the domain.
You'll initiate logins with Ctrl-Alt-Del. Note that you can either log in to the domain, or to the local machine without logging in to a domain by clicking the Options button to expose a drop-down menu listing your login choices.
Recipe 23.4, "Enabling File Sharing on Windows PCs," in Linux Cookbook, by Carla Schroder (O'Reilly) for more information on configuring Windows networking
Chapter 3, "Configuring Windows Clients," in Using Samba, Second Edition, by Jay Ts et al. (O'Reilly)