You should set up your DNS server to have Forwarders (your ISP's DNS servers), so when your DNS server received a DNS request it does not know (to a domain outside of your office on the Internet), your DNS server sends the request to the other DNS servers.
To Configure Forwarders:
1 - In DNS Manager, right-click the DNS Server object, and then click Properties .
2 - Click the Forwarders tab.
3 - Click to select the Enable Forwarders check box.
4 - In the IP address box, type the first DNS server to which you want to forward, and then click Add .
5- Repeat step 4 until you have added all the DNS servers to which you want to forward.
Please read Technet article "HOW TO: Configure DNS for Internet Access in Windows 2000 (Q300202)" for more info:
http://support.microsoft.com/search/preview.aspx?scid=kb;en-us;Q300202
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hope this helps