My experiences
I've tried a few things to get the Mac and PC to share a webcam session via MSN.
I tried Mercury MSN, which worked.. kind of... it kept crashing and the interface annoyed me to no end. So to the trash bin it went.
Squidcam worked but was a bit quirky but has the bonus of working outside of the chat/voice application you may be using.
I eventually settled (since I could find no better option) for using
aMSN with the
maccam drivers and I'm waiting for AdiumX to integrate voice and video. Apparently that is underway as a Google summer of code project.