At the moment we have good working solutions for OS X, though we did better with the discontinued iSight webcam than with the currently available lower quality embedded Mac webams. On the XP side things are much dicier, we really need either USB 3 or to do video compression on the camera (Firewire worked wonderfully; sometimes I really dislike USB).
So how can the good OS X solutions interoperate with quirky PC solutions? This blog post from 2007 and its comments is helpful:
Trying to Video Chat between Mac and Windows? | Times New RohanSo Skype is one option. The comments also mention iChat interoperability with the newest version of AOL's Instant Messenger and a beta Mac client for Windows Live Meeting.
...Finally we both downloaded Skype, and it just worked. We installed the application, created accounts, initiated video chats, and were chatting within minutes. (It is a well behaved Mac application to boot)...
I'll do some personal experiments and report back. My preferred solution would use iChat on OS X.
Update: I was able to install AIM on my XP box and connect my AIM/AOL username with iChat/MobileMe. The AIM client provides a fairly small video image. During AIM installation you have to be very careful to disable all other AOL-junk installs, and you may wish to delete all the plug-ins. You will be stuck with annoying embedded advertising that cannot be hidden.
I couldn't find much on OS X Live Messenger. There's now a corporate OS X Messenger client, but it requires an Office Communicator 2007 corporate server.
Skype is probably the only other option. I'll take a look at that next.
Update: Skype's high quality video solution is Windows only. Skype annoys me even more than AOL, which is saying quite a bit.
Update: A slightly dated tutorial on AIM and iChat videoconferencing. Some parts are up to date, others are obsolete. If I go forward with this project I'll have to write a post on setting up the AIM client for this use.
 Neat link by the way. A 1998 NYT article on how porn was going to drive videoconferencing. Well, it did -- almost to extinction. Turns out porn has a way of ickyfying an entire technology. Good lesson here -- also thank you NYT for letting Google trawl your archives!