PC to iMac video chat

I'm running a Sony Vaio with XP Home Edition, Windows Live Messenger and AIM
v.5.9.3861 and a Microsoft LifeCam, latest release, and Daughter has a new
iMac with it's camera and iChat.

We can text chat with either Messenger or AIM but can't seem to do video or
voice. I believe the iMac version of Messenger doesn't support video. Both
of us are using wireless but we've also directly connected to our broadband
modems in order to try to solve this. We've also turned off all the security
and firewall in a test.

Is this a Microsoft/Apple incompatibility issue or is there a trick I need
to know?

Thanks in advance