The Adprep page begins with:

Adprep

Prepares Windows 2000 domains and forests for an upgrade to Windows Server 2003, Standard Edition; Windows Server 2003, Enterprise Edition; or Windows Server 2003, Datacenter Edition. Among its tasks, adprep extends the schema, updates default security descriptors of selected objects, and adds new directory objects as required by some applications.

Syntax

adprep\{ /forestprep | /domainprep\}

Parameters

/forestprep	Prepares a Windows 2000 forest for an upgrade to a Windows Server 2003 forest.
/domainprep	Prepares a Windows 2000 domain for an upgrade to a Windows Server 2003 domain.
/?	Displays Help at the command prompt.

NOTE: See How do I upgrade Windows 2000 domain controllers to Windows Server 2003 and how to add new Windows Server 2003 domain controllers to Windows 2000 domains?