Hey friend, I use Iptux for chatting on my home LAN. Setup is quite simple. Merely install the client on each desktop (no server software needed the clients detect each other.) Iptux also includes file sharing capabilities.
On the ubuntu / debian based linux client machines use sudo apt-get install iptux
http://packages.ubuntu.com/lucid/iptux (appears to have been in the "universe" repositories since karmic)
On the windows client machines install ipmsg.exe (Iptux is a linux version of the windows ipmsg client or maybe the other way around.)
Also, be sure to open port 2425 on the firewall of each machine (and router if necessary) as that is the port that iptux / ipmsg uses.