One possible setup to Audio/Video conversation between Windows and Linux
First I will suppose you have audio and video working on both systems, Windows and Linux. Although I had audio working in Fedora 6, Ekiga could not record. I found the problem was related to ALSA/OSS emulation. I opened Volume Control, selected "File/Change device/OSS mixer" and toggled audio capture from microphone.
Then I had to deal with firewall settings. In my case, both systems connect to the Internet via ADSL modem with NAT. In this case, you will have to redirect some ports in the modem to your machine. I set my Linux station as default server in it's modem and I always connect from the Windows side to the Linux side. On the Linux side, I opened incoming connections on TCP ports 1720 and 30000 to 30010, and UDP ports 5000 to 5016. On doubt, stop your firewall to know if your problem is related to it.
If you run your modem in bridge mode, you won't need to redirect ports (or set default server), but you'll still need to deal with your firewall (iptables) settings.
Ekiga/NetMeeting is not my primary communication channel, so every time I want an Audio/Video conversation, I first contact via Instant Messenger (ICQ, Jabber, MSN) and then we both run Ekiga/NetMeeting.
On the Linux side, I ran Ekiga normally connecting to my account at ekiga.net. On the Windows side, I ran NetMeeting (conf.exe on Windows XP) without connecting to any server (I guess this doesn't matter). Then I send my Linux external IP address to the Windows side via Instant Messenger, and the other side direct connect NetMeeting to Ekiga's IP.
You can find out your external IP address just opening http://www.whatismyip.com/ in your browser.
That's it. Too much complicated? Well, that's how I did to have audio/video conversation between Windows and Linux. My mom surely appreciated it :-P
Comments are welcome.
Finally I've found a MSN Messenger client for Linux with Webcam support: aMSN. It's just click and go, I guess no further instructions are needed.