How can I block certain ports for outbound communication for a set of Computers running Windows XP when joined in a Microsoft AD Domain
Under Windows XP firewall only blocks incoming traffic. It does not monitor outbound traffic. for that functioality you would need to upgrade to windows 7. By default windows 7 also allows all outbound traffic but you can create a GPO that will turn on blocking all outbound traffic unless there is a rule to permit the traffic.