There is two chooses:
1) Use pidgin with Bonjour protocol (a P2P IM protocol)
2)(Harder) Or setup a local irc/xmpp server.
Take a look at setting up an Openfire installation (XMPP server... works
like any instant messaging client).. we use this at work between tech
teams, works cross platform and was very easy to setup.
http://www.igniterealtime.org/downloads/index.jsp#openfire
Good luck.