Greetings Mark,

Windows Messenger (5.x, 4.x doesn't include Exchange) supports only three ways of
connecting -- the public .NET Messenger network, a SIP-based communications service or an
Exchange IM server.

Since you're running on a restricted network (which I assume means no .NET Messenger access),
you'd need to use one of the other options, some examples of a SIP-based Communications
Service include the Live Communications Server from Microsoft (which may be a little over the
top for a small network), and the Formine RTC Server (which Vitali has already given you a
link to in this thread).

Another other option you might want to consider for peer-to-peer communication is LanTalk XP:
http://www.lantalk.net/lantalk_xp.php
____________________________________________
Jonathan Kay
Microsoft MVP - Windows Messenger/MSN Messenger
Associate Expert
http://www.microsoft.com/windowsxp/expertzone/
Messenger Resources - http://messenger.jonathankay.com