If you want a super-delux industry standard client-server instant messaging system, that's free, investigate Jabber. I currently deploy the ejabberd Jabber server and the Psi Jabber client. The server can be tricky to set up, but I have ejabberd running on a Linux server, integrated with Active Directory on Windows SBS 2003 and Windows XP on all the workstations. Works a treat. Popup messaging, instant chat, conference rooms, etc.