Mick,
A little reading will get you a long way. Open the
Windows (or MSN) Messenger main window. Click Help / Help.
Click All Topics. Click the + next to "Using MSN Messenger
from Behind Firewalls or Network Address Translators" and
browse the topics.
Here's a quick list:
Outgoing TCP connections (chat messages) are on port 1863
File Transfers are done via ports 6891 through 6900
Note
If you are using a network address translator (NAT), file
transfer will not work unless you have a Universal Plug
and Play NAT (UPnP NAT). If you are not sure what type of
NAT you have, check with your computer manufacturer or NAT
vendor.
Or, you can really load up your brain and go here:
http://www.microsoft.com/windowsxp/pro/techinfo/deployment/
natfw/default.asp
Regards,
Kevin