Samba is the best choice. I am running it as a Domain Controller, IP tables firewall, email server, web server etc. And it's rock solid. A word of caution: there is a little-known "feature" that requires you to create an account on the samba server for the XP workstations in the same way that you would create a regular user. The syntax is a little different though. If you don't do it, you'll never be able to log onto the domain or access the server.
Brian