Linux firewall / Win2000 network issue

I have set up a Mandrake 7.2 SNF firewall for a small Win2000 server network with about 50 clients. I cannot ping the Win2000 server nor can I ping the firewall from the server. They are on the same subnet and I have green LEDs on all NICs and switches, cables test good. I can ping lo and the other two NICs internally (has three), but nothing else. Any ideas on what I’m experiencing?