BorgChat - The best Lan chat software i have ever used, with a lot of options, and the best thing, it's Free completly here is the link, try it out http://borgchat.10n.ro//
Well all you need to doo is to set up your IP adress, Subnet mask, and DNS adress(the adress of 2003 server), and you need to change in network propertis that your computer is in domain, and not in workgrup!I hope i helped!