Windows Messenger

Windows Messenger allows real-time communication with other contacts who are using instant messaging (IM) services including those provided by the Session Initiation Protocol (SIP) as offered by Microsoft Office Live Communications Server. Windows Messenger also provides connectivity to the Microsoft .NET Messenger service and Exchange 2000 Server Instant Messaging Service.

System Requirements
Supported Operating Systems: Windows 2000 Service Pack 4; Windows Server 2003; Windows Server 2003 Service Pack 1; Windows XP 64-bit; Windows XP Service Pack 1; Windows XP Service Pack 2; Windows XP Tablet PC Edition

http://www.microsoft.com/downloads/details...ckInfoContainer

Windows Messenger 5.1

Windows Messenger 5.1 replaces Windows Messenger 5.0 and is the required version of Windows Messenger to take advantage of certain new features in Microsoft Office Live Communications Server 2005:
Federation (sharing IM and presence between two or more organizations)
Advanced architecture (higher availability and failover support)

http://www.microsoft.com/downloads/details...ckInfoContainer