Something that we have used inside of business firewalls for years is various forms of the Jabber messaging platform. There are server software packages that run on Windows, and there are both free and commercial options. Now available to run as a windows service no less.
This gives you messaging without having to open up to global messaging(Yahoo, MSN, GTalk ...) and all of the risks and distractions that come with those networks.
http://xmpp.org/xmpp-software/servers/
