DNS in WINDOWS 2000

Hi,
I have just installed windows 2000 with Active directory and setup DNS as the server which is say 172.16.8.1. I have also installed DHCP & WINS on the same server.
I have a firewall and leased line access to the internet.My ISP have given me 2 DNS IP numbers for the internet access which i added in the TCP/IP settings.
I can either access the internet or the internal local network at the same time but not both.

What am i doing wrong.

Could you pls help.

Charles.