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.
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)

For a complete list of fixes and changes in this version of Windows Messenger 5.1 please see KB article 899283.
If your organization uses Microsoft Windows XP Multilingual User Interface (MUI), download the Windows Messenger 5.1 Multilingual User Interface Package.

Download here

Ramesh wrote Is it lookign strange v5.1 ?, even we are using v8.1 and are waiting to next one

this is v5.1 of Windows messenger...

we are waiting for WLM 9.

i think there are therr messengers of Microsoft- MSN,Live,Windows.