You wont be able to "talk" to him unless you know sign language.
There are a couple of Mac apps that support webcam functionality with MSN users, but neither of them support audio. The two apps are Mercury (
http://www.mercury.to) and aMSN (
http://amsn-project.net).
If you're just interested in text IM then it's possible to do this in iChat, but not with an AIM or .Mac ID. You'll need to set up iChat to use XMPP (or Jabber as some call it). There are many XMPP servers out there to choose from but my recommendation is Google's "Talk" service. If you have a gmail address you can already access their gtalk service too. Instructions for setting iChat up to use it are available at
http://www.google.com/support/talk/bin/answer.py?answer=24076
After that you have to go through a convoluted process of setting up a transport to the legacy MSN IM network. The process is described at
http://allforces.com/2005/05/06/ichat-to-msn-through-jabber
Alternatively if you just want to audio/video chat with your Windows friend you could both install the horrible, proprietary Skype (
http://www.skype.com) which should solve your problems.
