Promote it to a domain controller - run DCPROMO and follow instructions. NOTE: if the new server is Server 2003 R2 and you DO NOT have a Server 2003 R2 domain controller already, you'll need to run ADPREP from the second CD on the existing DC to prepare Active Directory for the new server. This is ONLY REQUIRED if your new server is running a newer version of windows than your newest DC.
[If using R2 release of Windows 2003] Extending Your Active Directory Schema for New Features in Windows Server 2003 R2
http://www.microsoft.com/downloads/details.aspx?familyid=5B73CF03-84DD-480F-98F9-526EC09E9BA8&displaylang=enHere are some links that should help you in replacing existing domain controllers and/or transferring services and Active Directory functions to another/new system:
How can I transfer some or all of the FSMO Roles from one DC to another?
http://www.petri.co.il/transferring_fsmo_roles.htmHow To Create or Move a Global Catalog in Windows 2000 (essentially the same for 2003)
http://support.microsoft.com/?kbid=313994How to move a DHCP database from a computer that is running Windows NT Server 4.0, Windows 2000, or Windows Server 2003 to a computer that is running Windows Server 2003
http://support.microsoft.com/?id=325473How can I move a DHCP database from one server to another?
http://www.windowsitpro.com/Article/ArticleID/13473/13473.htmlHow can I move DNS from one Windows 2000 Server to another Windows 2000 Server?
(Two Related if going 2000 to 2003 - read both before moving)
http://www.jsifaq.com/subG/TIP3300/rh3357.htmhttp://www.jsifaq.com/SUBN/tip6700/rh6731.htm