Which port do what

hHi
I'm sitting in a network with 1500 users and have seen a warning about using
messenger. The articel said that you can get hackers laying virus trough
messenger, but we have only open port 80 in the firewall. Is'nt it only chat
that are going through port 80 or can you now make filetransfers through port
80. Anyway wich port doing what??????
--
Speednissen
Denmark