Video Conference With Mac 10.4 And Yahoo Messenger to Pc


I like to make Video conferences with my friends in Germany.
I downloaded the Yahoo current messenger for Mac OS and use a Isight camera. Unfortunately most of my friends use Windows PC. During a video conference I receive only a bad picture of them. I could not here them and they could not here me. Also I did not see a microphone or telephone symbol at my messenger window.
My microphone works, because my mac follow my voice commands.
Did I make a mistake with the settings?
Did Yahoo do not support Mac/PC video conferences?
How can I get in contact with my friends?