I have a W2000 server running SP4, I ran ADPrep /forestprep and ADPrep /domainprep, they both seem to run fine with no errors. (I ran ADPrep right off of the W2003 CD from the /i386 dir using a command prompt on the W2k server.) I did have to give the administrator the schema admin and enterprise admin group permissions to get these commands to run.
When I run DCPromo, it fails with the error telling me to run ADPrep to prepare the domain. I waited 15 minutes and re-ran DCPromo, I get the same error.