Issue: When running dcpromo on a 2008 r2 server in a 2003 domain, you’re prompted to run adprep /forestprep from the 2008 r2 installation media. When you run adprep /forestprep, you’re prompted that this is only available to existing domain controllers. Solution: You need to run adprep /forestprep and adprep /domainprep on an existing 2003 domain controller from the 2008 r2 installation media. If your 2003 server is 32-bit, run your need to run adprep32. Given your installation media is on D:\ if 32-bit:

support\adprep\adprep32 /forestprep
support\adprep\adprep32 /domainprep

if 64-bit:

support\adprep\adprep /forestprep
support\adprep\adprep /domainprep

You should now be able to run dcpromo on your Windows 2008 r2 server.