Windows Messenger is a proprietary instant messaging client included in
Microsoft's Windows XP operating system. It is also available for Windows
2000 and Windows Server 2003, but will not be included in or supported on
Windows Vista.

Windows Messenger should not be confused with the similarily-named Windows
Live Messenger (formerly known as MSN Messenger), which is a separate instant
messaging product available for download from Microsoft. It is also not
related to the Messenger Service, which is a component of Microsoft Windows
itself.

Overview
Windows Messenger was introduced with the release of Windows XP, and is
installed and enabled by default. It has a variety of features, such as
instant messaging, presence, file transfer, application sharing and
whiteboarding. Later versions added "ink" support and integration with Live
Communication Server. Windows Messenger integrates with Microsoft Exchange,
Microsoft Outlook, Microsoft Outlook Express, and the Remote Assistance
feature of Windows XP. Windows Messenger also integrates with Media Center in
Windows XP Media Center Edition. Windows Messenger can communicate with the
Exchange Server 2000 Instant Messaging Service and the .NET Messenger Service.

Development of Windows Messenger was halted after version 5.1, in favor of
Windows Live Messenger and Office Communicator. Plugins for Windows
Messenger, such as accessing the inbox of a Hotmail account, are no longer
available; users are instead encouraged to download Windows Live Messenger or
Office Communicator, depending on their needs.

User interface
The Windows Messenger user interface is devoid of a lot of the "clutter"
seen in Windows Live Messenger. Winks, nudges, and custom emoticons are all
unsupported, and the main user interface is more closely based on the
standard Windows style.