I have a windows box on the network that uses MSN to video chat with someone across the internet. What is the securest way to configure the firewall to allow this?

Which ports would be required to be open?

The windows box would only be video chatting with one person. What about only allowing connection to the MAC address of the card on the internet and block all other incoming connections on those ports?

I'm really new to IP tables.

The windows machine is running XP Pro and the firewall will eventually be running Gentoo as soon as I get it up and running.

Thanks!