Voice and vedio chat behind XP ICS and ICF

I set up an XP professional PC as Internet Gateway
connecting to cable modem.

However the voice and vedio chat on other computers are
not working at all. The gateway PC is the only one that
can have audio and vedio feature.

This the warning Messenger threw to me:

Warning: Your computer, Internet provider, or network uses
some type of Network Address Translator (NAT). Some NATs
prevent successful audio or video conversations.

Does any body know how to solve this problem?

Thanks in advance.

GATEWAY: XP PRO

CLIENTS: XP PRO, 2000PRO, 2OOO ADVANCED SERVER, WITH
DIRECTX 9.0 OR DIRECTX9.0A