I consider myself a person who knows my way around osx, but I didn't even know ichat worked with AIM on windows... try to use http://www.adiumx.com/
for just text chats betwee windows and mac... otherwise if you know the video chat is technically possible and it says on the internet you can do it... I suggest you search on the internet for what ports AIM uses than search on how to open ports on your router model (use google to do this)... Note working with routers isn't for the faint of heart, but is easy once you take the time to learn. To get to your routers login screen just put the following url into your web browser... "192.168.1.1" If it asks for a password and you never put one in... just search on google for your models default password and login name.
Good luck... If you want to avoid all of this and do text, audio, and video between any computer platform easily and for free, I use http://www.skype.com/