Here are a few great articles that discuss how to restore Active Directory. This is something you want to practice and document in your format so that when and I mean WHEN it happens you are prepared.
Windows Server 2003
Performing a Nonauthoritative Restore of a Domain Controller
Performing an Authoritative Restore of Active Directory Objects
Windows Server 2003 and Windows 2000
How to restore deleted user accounts and their group memberships in Active Directory
How to perform an authoritative restore to a domain controller in Windows 2000