The ports for Yahoo and MSN messengers are:
Yahoo Messenger = 5000 to 5010 and 5050
MSN Messenger = 1863
The other option with msn messanger is
Blocking all Outgoing TCP connections to the host
207.46.110.*
207.46.104.*
Yahoo messenger is
216.136.223.*
216.136.224.*
The other option is to put the following lines in the host file.
127.0.0.1 https://login.passport.com
127.0.0.1 login.passport.com
127.0.0.1 messenger.msn.com
127.0.0.1 messenger.msn.ca
127.0.0.1 www.msn.com
127.0.0.1 msn.messenger.net
127.0.0.1 hotmail.msn.com
127.0.0.1 loginnet.passport.com
This will allow you to control per PC who has access and you can add as many sites as you want. If the users are novice this will work or you can lock out the host file. You will have to similar thing for yahoo sites.
Of course you can purchase appliances like the ones from St Bernard and get total control over which websites as well as what times and by whom. This may be more than what you want to pay. The Enterprise edition of Symantec for a software solution.
--Hemant
