This is a step by step Video Tutorial on how you can setup an Enterprise Class Instant Messaging system in just about 180 secs.

Ejabberd Instant Messaging

From EJABBERD site:

ejabberd is a distributed, fault-tolerant technology that allows the creation of large-scale instant messaging applications. The server can reliably support thousands of simultaneous users on a single node and has been designed to provide exceptional standards of fault tolerance. As an open source technology, based on industry-standards, ejabberd can be used to build bespoke solutions very cost effectively.