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MSN Messenger is a freeware instant messaging client that was developed and distributed by Microsoft in 1999 to 2005 and in 2007 for computers running the Microsoft Windows operating system (except Windows Vista), and aimed towards home users. It was renamed Windows Live Messenger in February 2006 as part of Microsoft's Windows Live series of online services and software.
MSN Messenger is often used to refer to the .NET Messenger Service (the protocols and server that allow the system to operate) rather than any particular client.