A very good friend of mine finally managed to sort out PC (using AIM) and Mac (using iChat)...
for those who have tried it and had problems he recommends the following on the WinXP machine:
Windows XP internal firewall: Updating a computer running Windows XP to SP2 (Service Pack 2) may cause Mac iChat AV to AIM 5.5/5.9 PC video and audio connections to fail because, by default, the firewall created by SP2 blocks UPnP communications from the Internet. If the SP2 firewall is enabled: Go to Start > Control Panel > Security Center > Manage security settings for: Windows Firewall > Exceptions tab> add or check UPnP Framework as an Exception. Select UPnP Framework and click the Edit... button. Be sure that both the TCP 2869 and UDP 1900 ports are checked and listed as 'Any' under "Scope."
If the Scope of each port is listed as 'Subnet,' click the Change scope... button, and select the 'Any computer (Including those on the Internet)' radio button. Do this for both ports, and then save the changes by clicking the OK buttons.