xp can't be a domain server, xp can only be a client of a domain server, unless you find some third party stuff i suppose...windows 2000 server or windows server 2003 would be the only way to go.
Like posted previously, XP can not be a domain server. Are you sure you need a Domain server? What is it that you are trying to accomplish? XP can be easily networked to share it's resources and you may not even need a Domain server.
pao_pao, go get a copy of Windows 2000 or 2003 Server. That's the only way you will be able to create a Windows domain. XP Professional and 2000 Professional can join the domain. XP Home can't.