MSN Web Messenger lets you talk online and in real-time with friends and family using just a web browser! Use it on any shared computer - at school, at work, at a friend's house or anywhere you can't install the MSN Messenger software.
MSN Web Messenger is an easier and more convenient way to enjoy many of the features in MSN Messenger without having to download and install the MSN Messenger software.
You must have the following to use MSN Web Messenger:

• A web browser: Microsoft Internet Explorer 5.0 or later, Netscape 7.1 or later, or Mozilla 1.6 or later, running in Microsoft Windows.
• An Internet connection (56 Kbps or faster recommended)
• A Microsoft® .NET Passport. If you have a hotmail.com or msn.com account, you already have a Passport.
• Popups enabled for this web site if you are using popup blocker software like the MSN Toolbar

Note: 
MSN Web Messenger is the official way to communicate with your MSN Messenger contacts using a web browser.
To use Web Messenger, your web browser must allow pop-up windows. For information about allowing pop-up windows, see the instructions for your pop-up blocking software.

What is the difference between Web Messenger and Messenger?
MSN Messenger is a fully featured instant messaging program that you install on your own computer or one you have permission to install on. MSN Web Messenger enables you to quickly and easily use basic instant messaging features on a web browser on any computer without installing any software.  
 

Official Link to MSN Web Messenger!