Jabber servers are small and easy to install on any windows/mac/linux machine. Say you have 25 machines on your internal network (192.168.1.100 thru 192.168.1.125), you could install the Jabber software on any one of those machines (say .105), install pidgin on every machine, and have pidgin connect to 192.168.1.105 for chat. That way, all your chat sessions will remain inside your internal network.
If you are only referring to chat between two people on a network, there are simpler tools available that'll connect two PCs for chatting. I can't remember any right now, but I'm sure someone can recommend something.