Windows 2000 as router

Hi,
I have the following case.
windows 2000 with 3 nic.
1- first nic has 10.0.0.1 IP for internal lan
2-second nic has 213.x.x.x internet connection
3-third nic has 172.0.0.1 DMZ zone.
the problem is the windows 2000 cannot handle the routing. no one in the LAN can ping and pc at DMZ they only can ping the third nic adderss
can you tell me how to make w2k handle routing isuue I did alot configration but didnt work