Should be run run adprep when you want to install a higher version of windows controller in an existing forest

Before you can introduce Windows Server 2008 domain controllers into existing Windows 2000 or Windows Server 2003 domains, you must prepare the forest and domains with the ADPREP utility. ADPREP.exe is a command-line tool that extends the Active Directory schema, and updates permissions as necessary to prepare a forest and domain for a domain controller that runs the Windows Server 2008 operating system.

Note: ADPREP was also available in Windows Server 2003 and Windows Server 2003 R2. In Windows Server 2008, ADPREP follows the same logic and performs similar tasks to prepare for the upgrade to Windows Server 2003 or Windows Server 2003 R2, which is available on the Windows Server 2008 installation disc in the \sources\adprep folder.

The adprep includes the following commands:               
You should run "adprep /forestprep" on the the Schema Master of the forest;

You should run "adprep /domainprep" on the Infrastructure Master of each domain;

you should run "adprep /domainprep /gpprep" on the Infrastructure Master of each domain;

You should run " adprep /rodcprep" on the Windows Server 2008 regular DC in order to enable the installation of the first RODC.

In order to run ADPREP, insert the DVD media of Windows Server 2008 into the DVD drive of the appropriate Windows 2000/2003 DC, which should be the appropriate master of the forest. If

you're prompted to install Windows Server 2008, do NOT install it. Close the window instead. Browse to the
\sources\adprep directory. Open a Command Prompt window (Click Start > Run> CMD > Enter), and drag the ADPREP.exe file to the Command Prompt window. Run the appropriate commands like:

"C:\Documents and Settings\wayne.wong.WESOFT>D:\sources\adprep\adprep.exe /forestprep"

For the details, you can refer to the following links:

http://www.petri.co.il/windows-server-2008-adprep.htm