it is blocking yr internal IP's/netmask at the firewall
i dont know PC cillin firewall...
doesnt it have like a 'enable NAT' function? this way you shouldnt get problems, cause it would use yr IP from the computer that is connected to the cable modem to connect to the internet, n the firewall shouldnt block it
or an 'advanced rules' section in the proggy?
where you can be more especific about wich IP should the firewall block, or not, and in wich direction?