|I would start from scratch and do a clean install of the operating system. While installing, or immediately after, statically assign an IP address to the server as ijack said.|
If your router is setup something like this:
Router's LAN IP: 192.168.1.1
Subnet Mask: 255.255.255.0
DHCP = Enabled
DHCP Scope = 192.168.1.100 to 192.168.1.199
Then I would use something like the following TCP/IP settings for the server:
Default Gateway: 192.168.1.1
Then, once it's setup and communicating, I would do any/all necessary OS updates. Once you're finished that, then run dcpromo to make the server a DC and allow it (Windows) to setup DNS during the promotion.
When you're finished, ensure you have DNS forwarding setup on the DC's DNS server. You'll want to forward to either 192.168.1.1 (the router) or to your ISP's DNS server(s). Personally, I would use the ISP's DNS for the forward.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.