Make sure that you can log on to the schema master with an account that has sufficient credentials to run adprep /forestprep. You must be a member of the Schema Admins group, the Enterprise Admins group, and the Domain Admins group of the domain that hosts the schema master, which is, by default, the forest root domain.Source
I was able to fix by going into Component Services on 2003 server, right click 'my computer', properties, on 'default properties' tab 'enable distributed COM on this computer' was unchecked. I checked it and reboot both servers, I was then able to promote the 2012 server to DC, adprep went thru with no issues.EE PAQ 28168026
-saige-