SUP!!

Im trying to set up a linux firewall for my Lan (linux 7.1) I have 2 win2k machine behind the the linux box. I setup eth0 as the gateway for the "win" machines and eth1 for the internet on my linux box.Now all my machines can ping each other. BUt when i try to ping www.yahoo.com their is no reply- but it does say transmitted xx packets --0 packets received?? I try to surf the net and that obviously doesnt work either. Sooo i tryed to ping my ISP gateway and that works and same with their dhcp. Anybody have an idea what i forgot to setup or did worng? I also tryed nslookup and it resolves the names--- Any ideas!!! Thanks

P.S.> I just a newbie to linux