First, let’s clarify what "messenger" we are talking about. Clarification is necessary because Windows actually has three "messengers." First there’s MSN Messenger and then there’s Windows Messenger and finally there’s the Windows Messaging service.
MSN Messenger is Microsoft’s end-user oriented instant messaging (IM) service.
Windows Messenger is also an IM service but its intended use is within corporations. It’s only available for Windows XP and some versions of Win2k. Unlike MSN Messenger, Windows Messenger is quite tightlyintegrated into key Microsoft products like Outlook, Exchange and Windows XP itself.
Both share the same icon in Windows. Both make use of the same .NET contacts and are, in a general sense, interchangeable. That’s the way I’m going to treat them here.
Then there’s Windows Messenger Service. Unfortunately, some folks incorrectly refer to this as "Windows Messenger" which just adds to the confusion. In fact, Windows Messenger Service is not an IM product at all. It’s a network messaging product that was introduced with Win2K. Its intended use was to provide popup information windows in a network environment. For example, when Tech Support wants to broadcast a message to all users like "the system is going down in 10 minutes."