First check that DNS is installed...
Control Panel >Add Remove Programs > Add/Remove Windows Components
Ensure the Networking Services check box is checked, Click on Details and ensure the Domain Name System (DNS) is checked. If it isn't, check it and click OK. Windows will then install DNS on the server. You will then be prompted to insert the Windows 2003 CD to complete the installation.
Once installed, click Start > Administrative Tools > DNS
Right click on the Forward Lookup Zones and select New Zone...
Primary zone, Next >
Zone name: yourdomain.com, Next >
Next >
Do not allow dynamic updates, Next >
Finish
Right click on the yourdomain.com zone you just created.
New Host (A)...
Name: ns1 (for ns1.yourdomain.com)
IP address: (this is the external IP address assigned to you by your ISP)
Click Add Host
Double click on the Name Server (NS) entry in the zone.
Remove the existing FQDN and Add a new one.
Click on Browse and browse to the Host (a) entry you created above (ns1.yourdomain.com).
Click OK.
Done... the name servers are set up.
To add domains that resolve on this name server, you would add another forward lookup zone.
As above, you need to edit the Name Server entry, remove the existing FQDN and Add a new one. As above, you also need to click on Browse and browse to the original Host (A) entry that you created (ns1.yourdomain.com).
Once you have done that, you can add more host (A) entries for this zone.
Here's a screenshot of how it should look...
I've edited out a few values, hence the white spaces... but hopefully you get the idea.