Once you have installed DNS you will have a DNS option in Admin tools and also on the startup screen when 2003 loads up. Click the "manage this DNS server" from the splash.
Click on the little computer icon in the left pain and in the right you will see a list of options. One of those will be called "Forwarders"
In here add the routers ip address as the main one and your ISPs DNS are the secondary.
THen any IPs that the server cannot resolver it will question the router/ISP.
If you do not do this when you log on the pc will try to resolve the host name/passwords with the ISP and not the server and can take ages to timeout and continue to load!!