There are some web-based alternatives that don't require the installation of an MSN Messenger client. So as long as network administrators don't block these particular sites, you will be able to chat to your MSN contacts (although not with all the bells and whistles of the normal MSN Messenger):
1. WBMSN
WBMSN ("Clientless Conversations") is a good and popular online MSN Messenger script by Jared and comes very recommended:
Everywhere MSN or eMSN is a webbased application created by Paulo Taylor and enables users to connect to MSN through a desktop browser or a mobile phone (or Palmtop) using cHTML/xHTML or WAP 2.0.
3. Piglet IM
Piglet IM is a 100% webbased MSN client by Jeroen Geerdink and tipped to my by Brian Turchyn. It's still finding its way on a new server, but it's definitely worth checking out.
Piglet IM http://www.mess.be/_scripts/fclick/fclick.php?fid=piglet
4. MSN2Go
MSN2Go is the world's first free web-based MSN Messenger client to be created in the form of a Java applet. It's not another HTML/JavaScript hybrid, but a true Java applet that works equally on any Java-enabled browser. That means no page-refreshing whenever a user changes status, no JavaScript kludges, and no DHTML glitches and artifacts.