Windows Messenger is designed for business use, while MSN Messenger is designed for everyday consumer use. You’ll find more ‘fun’ features in MSN Messenger, such as display pictures, custom emoticons, winks and lot’s of customization options, while with Windows Messenger you’ll find it is more simplicit and to the point — communication, without lots of ways of expressing yourself.
MSN Messenger uses Mobile Status Notification Protocol (MSNP) protocol to connect to the .NET Messenger Service network, it contains advertising, and is updated manually by users. Windows Messenger works with both .NET Messenger Service, but is more focused on usage with Microsoft Live Communications Server using the Session Initiation Protocol (SIP)/SIP Instant Messaging and Presence Leveraging Extensions (SIMPLE) protocol, it does not contain advertising, and is updated through the Windows Update mechanism.
On the Windows Messenger Resources Page at Microsoft Office Online, you can download a document which describes the differences really well, download WM vs MSN Messenger.doc from Microsoft download center.
The main points from the document are:
- Windows® Messenger is designed for use in a business environment. With a single application, you can communicate with contacts via your organization’s Microsoft Office Live Communications Server 2005 server, as well as with contacts that use MSN Messenger.
- MSN® Messenger is designed for use in a consumer environment. It only connects to contacts, such as friends and family, who use the publicly available MSN .NET Messenger Service. Communications are not encrypted, as they are with Windows Messenger.
After reading, I’m even more confused.