Most people configure production boxes in a lab before they bring them out onto the production floor. If you do this, your lab is most likely not connected to the corporate LAN. This was the case when I set up my first corporate Windows 2000 Active Directory server.
When you set up Active Directory and you do a DC Promo, if you don't have DNS installed on that box, DC Promo will install it for you, as well as set it up. What you don't realize is that it will set it up as a DNS ROOT SERVER. The significance? ROOT SERVERS can't be set up as DNS forwarders. This is a problem in a pure Windows 2000 environment (workstations and servers).
If you don't put the Windows 2000 DNS server as the DNS server of choice in each of the Windows 2000 Professional boxes (i.e., you put your ISPs DNS servers instead), logging on will take 2-3 minutes as the workstations try to register with DDNS and fail to do so. You will also have an event logged in the event viewer.
If you put your domain's Windows 2000 DNS server as the DNS of choice for the clients, the clients will be able to log in quickly, But that DNS server will be unable to resolve Internet addresses. (It is a root server for your domain because of the way Active Directory set it up.)
So how can you fix this troublesome problem? Make your DNS server a NON-ROOT server, or a DNS forwarding server. But this is not as direct as it seems...it took me a day in the lab to figure it out.
How do you do it? In DNS manager, delete the "." domain (minus the quotation marks, of course). Restart the DNS server service and that's it. Now you can put your ISP's DNS server entries in the FORWARDER settings on the Windows 2000 DNS server (found on the properties page of the DNS server itself through DNS manager). Anything that your domain can't resolve locally (within your domain) will be forwarded to your ISP's DNS server.