This document describes how to use MS Windows Messenger v4.6,v4.7 and v5.0+ with Session Initiation Protocol (SIP) to establish live communication sessions via instant messaging, voice or video. Although MS provides its own SIP server, known as MS Live Communications Server, this article is based on experiences during the usage of MS Windows Messengers with SIP Express Router (SER).

Microsoft has the merit of developing one of the first free client applications that supported SIP -- MS Windows Messenger. The application is included in the standard Windows installation kit, for sure in Windows 2000 and XP.