I'm going along with this comment I found on cnet - it's on the Yahoo Messenger download page:
"If you are just looking for a audio video communication tool, this si the best. MSN 6.2 is now having audio problems for those who use firewalls and router and MSN help unnecessarily recommends opening thousands of UDP port to enable audio, which I feel is totally unnecessary. If you go with Yahoo Messenger you wont need to fiddle around with your router and firewall setting and besides Yahoo Messenger has audio video conferencing capabilities which means you can talk to all your friends and relatives world wide in real time and see them as well, depending on how much ram and CPU you have. "
source
I'm going to try Yahoo messenger l8r on today.
In the interim I tried MSN messenger over the two PCs I have at home (linked together by wifi) and both video and audio worked ok. One machine is win2k and one is win98 - both with MSN messenger 6.2. It does still have to go over the Internet but I guess the internal network facilitates the transmission of data somewhere.
Fingers crossed for Yahoo messenger - I'dl love it to work, it's a great facility..