You will have to setup an user account for every computer that shall connect to your server as described in the howto.
Then you will have to reconfigure your windows workstations to connect to your server as domain controller. This is configured under start > settings > controlpanel > system on the tab computer name.
Be aware that you need Windows XP professional, windows 2000 or windows 98 for this setup. The setup will not work with Windows XP Home edition.