You want to promote a new domain controller using a backup from another domain controller as the initial source of the directory contents (DIT) instead of replicating the entire DIT over the network.
You first need to back up the system state of an existing domain controller in the domain the new server will go in. This can be accomplished by running the MS Backup utility found at Start Programs Accessories System Tools Backup.
Once you have a good backup, you then need to restore it to the new server, which can also be done using MS Backup. You should restore the files to an alternate location, not to their original location.
Next, run dcpromo with the /adv switch from a command line or Start Run:
> dcpromo /adv
After the dcpromo wizard starts, select Additional Domain Controller for an existing domain and click Next.
Under Copy Domain Information, select From these restored backup files, browse to the backup files, and click Next.
Enter credentials of a user in the Domain Admins group in the domain you are promoting the domain controller into and click Next.
Choose the folders to store the Active Directory Database and Log files and click Next.
Choose the folder to store SYSVOL and click Next.
Enter a Restore Mode password and click Next.
Click Next to start the promotion.
Being able to promote a domain controller using the system-state backup of another domain controller is a new feature in Windows Server 2003. With Windows 2000, a new domain controller had to replicate the entire DIT over the network from an existing domain controller. For organizations that had either a really large Active Directory DIT file or very poor network connectivity to a remote site, replicating the full contents over the network presented challenges. Under these conditions, the promotion process could take a prohibitively long time to complete. Now with the dcpromo "install from media" option, the initial promotion process can be substantially quicker. After you've done the initial install from media (i.e., backup tape or CD/DVD), the domain controller will replicate the changes since the backup was taken.
Recipe 16.1 for backing up Active Directory and MS KB 240363 (HOW TO: Use the Backup Program to Back Up and Restore the System State in Windows 2000)