In order for the XP machine to join the Server 2003 domain the server must be online at the same time as the XP machine. So as long as you have enough resources to run both virtual machines simultaneously and you have properly created an Active Directory Domain on the Server 2003, then you can easily join the XP machine to the domain using Domain Admin credentials.
Be sure also, that you have installed DNS on the Server 2003 and that the XP Machine's DNS settings point ONLY to the Server 2003's IP address.
Further details on how to install AD on Server 2003:
Further details on how to join the workstation are here: