You want to create a new domain that may be part of an existing domain tree or the root of a new domain tree.
Run dcpromo from a command line or Start Run.
On a Windows 2000 domain controller, select "Domain controller for a new domain" and then you can select one of the following:
Create a new domain tree Place this new domain tree in an existing forest
Create a new child domain in an existing domain tree
On a Windows Server 2003 domain controller, select "Domain controller for a new domain" and then you can select one of the following:
Domain in a new forest
Child domain in an existing domain tree
Domain tree in an existing forest
dcpromo can also be run in unattended mode. See Recipe 3.4 for more details.
The two options dcpromo offers to create a new domain are adding the domain to an existing domain tree or starting a new domain tree. If you want to create a new domain that is a subdomain (contained within the same namespace) of a parent domain, you are creating a domain in an existing domain tree. If you are creating the first domain in a forest or a domain outside the namespace of the forest root, you are creating a domain in a new domain tree.
Each domain increases the support costs of Active Directory due to the need for maintaining additional domain controllers and time spent configuring and maintaining the domain. When designing an Active Directory forest, your goal should be to keep the number of domains that are necessary to a minimum.
Recipe 3.1 for promoting a domain controller, Recipe 3.4 for automating the promotion of a domain controller, MS KB 238369 (HOW TO: Promote and Demote Domain Controllers in Windows 2000), and MS KB 255248 (HOW TO: Create a Child Domain in Active Directory and Delegate the DNS Namespace to the Child Domain)