Yahoo messenger on client side

Dear All:

I have one server in which windows 2000 server & ISA server 2000 installed on cache mode. In my client side windows 2000 professional & yahoo messenger are installed. Problem is that when my client side open web cam option in yahoo messenger the message appear “You are behind the firewall”. In the client side no firewall is installed. Please give me a advise how can my client see the web cam. I will be very thankful to your.

Regards,

Mansoor