windows adding dc 2008 -2003 ? is this possible to do

The root dc is on a 2003 server, im trying to add a new one with a 2008 server. I keep getting the notification to run adprep /forestprep on the server the root is on. But i cant, it cant find this, so i downloaded win server 2003 iso and mounted it and tried again and still nothing. so right now im stuck and id appreciate help…

btw i am adding this for a backup domain controller