iChat uses the AIM network, so you are using an AIM name or an AOL email address or a Mobile Me email address, your chat partner will use any client (read: chat program/application) that uses the AIM network. Instruct them to use AIM software and sign up for an AIM acct for free. Jabber or a Google talk acct will also work. Here are the (dis)advantages: When you communicate with other iChat (Tiger and later OS) users you may have up to 4 person video chat, and 10 in audio, but when talking to PC land it will only be 1 on 1. An advantage to using your mobile me name (on both sides) all modes of communication are encrypted (as long as you set up ichat encryption), but never encrypted when talking to PC land. One last thing I can think of off the cuff is that if you are both using leopard you can also share screen and use iChat theater, both really awesome features which don't even exist on PC side. (Not with AIM chat I mean)

So getting to a direct answer in a round-a-bout-way, They need to have an AIM name and the AIM client, you both must have high speed internet, one of you must have the other on a buddy list, and that should cover it. Setting up their cam to work with AIM is out of my realm a bit. Sorry I don't do windows. ;)