Coppyed directly from an ad for some messenger blocking software:
Windows Live Messenger is an updated version of MSN Messenger and uses similar ports. As part of a forum to Microsoft's web site and other web sites (e.g. www.cyberphaze.net - not currently online) it seems that Windows Live Messenger uses:
Messenger server: port 1493, 1542, 1863, 1963, 80 TCP and 443 TCP
File Transfer/Sharing Folders: local: 1544 and 6891 - in fact 6720-65535 TCP from one source
Messenger Update: remote: 80 local: 1457
Remote Assistance If available): 3389 TCP
Audio: local: , 1556, 11771, 13803 and generally 5004-65535 UDP
Remote Desktop and whiteboard: local/remote: 389, 522, 1503, 1720, and 1731
Launching Games: 80
Video Conference: TCP 9000-9999, 5004-65535 UDP + 80
Sign-In: remote: 443 local: 1484, 2400
Note - blocking TCP port 80 will stop users accessing web sites using Internet Explorer and other browsers.
Now it can USE all of these ports but you only need to port foward a small range, the systems are dynamic and if one port is open it will try to use that one.