I've done Mac/Win videoconference with my dad. I'm using an iMac intel, and he's got an IBM Thinkpad running WinXp. I put a $35 Logitech ball camera on his laptop, installed AOL AIM, had to tweak a bit, and voila, worked like a champ.
Here's a great tutorial:
Ichat/Win Tutorial
Note that it recommends that AIM v5.9 is what to use, NOT 6.0. That site provides a link to the older v5.9.
Because I was installing the camera on a low-powered Thinkpad (with limited USB ports), it took a fair bit of tweaking the camera to get audio and visual.
The Mac, on the other hand, worked as soon as you opened iChat.