MSN Messenger will try port 80 when it's original port is blocked (sneaky bugger!). I doubt you blocked port 80, since it's the port for HTTP (browsing). Essentially, nobody would be able to browse the internet any longer.
See Blocking Chat program
thread1117-810210
It explains which IP's you can block to stop MSN Messenger as well as other chat softwares.
"In space, nobody can hear you click..."