Couple of points:
1. Unless you have a prior arrangement with your ISP, most ISPs are set up to handle Active Directory (AD)-generated DNS traffic.
2. AD gets downright unhappy if it doesn't get the DNS information it wants.
How are your IPaddresses currently assigned? Either your ISP has given you a block of public IP addresses, the ISP is providing dynamic addresses (through DHCP) for a certain number of computers, or you have some sort of router/ firewall that is providing DHCP services. I will assume the last is what you have.
How are you currently sharing the xDSL line? Either you are running Internet Sharing on your server, or you are using a hub/switch connected to the router mentioned above. Again I will assume youhave the second of the two.
Based on these assumptions, here is what to do:
1. From your router, you will need the following information: the routers internal IP address (your default gateway), and your ISPs DNS server IP address(es).
2. Stop/disable the DHCP server function of your router.
3. Give your server a static IP address in the same range as the internal address of your router (probably 192.168.1.X).
4. Run dcpromo installing DNS along the way. Make sure your AD/DNS domainname does not include any of the Internet upper-level domain names (.com, .org, .net, etc.). Use something not recognized by the Internet (such as .local, .private, or something similar). This way you don't have to register your domain.
5. Have your server point to itself for DNS resolution. In your DNS settings, place your ISP's DNS server IP address in the Forwarders section.
6. Install DHCP. Create a scope using the same IP address range as both your DNS server and router. Use your internal DNS server's IP address for the DNS server in Scope Options.