NetChat

From Barefoot Productions:

NetChat is a local network messaging and communications system for Windows which does not require a server to be running for use. It allows you to chat in real-time with other users on a secure and private local network connection. Getting started with NetChat is very easy--just install the software on two or more computers on the local network and you may begin chatting in real-time with other users. Features include the ability to change font and text color, including emoticons and web links in messages, message logging and easy installation. Simply install on each computer (no server install or administration) and start communicating over your local network. Instant messaging has never been easier to configure and use.

Version 2.6 includes message logging and updated user-interface enhancements.