Windows Messenger
Windows Messenger is an instant messaging client from Microsoft that can communicate both via SIP and .NET Messenger Service. It is pre-installed in Windows XP, and versions 5 and higher are available for download for Windows 2000.It should not be confused with MSN Messenger (which is limited to .NET Messenger Service), although it looks very similar. MSN Messenger's free download availability is less limited. Windows Messenger is integrated with Microsoft Exchange, Microsoft Outlook, Microsoft Outlook Express, and the Remote Assistance feature of Windows XP.
It has a variety of features, such as instant messaging, presence, file transfer, application sharing and whiteboarding.