Go back to my link and look at the sublinks too.....

About 6 months ago I redid my AD domain at home and because I have a pretty slow box for the server and I'm an impatient SOB I forced a reboot after the box appeared to hang during a dcpromo.... In response to a specific error message I found a way to force the server to believe that it is no longer a member of an AD domain on M$' web site... Unfortunately it was one of those accurate but cryptic error message and I have no clue what it was. There is a registry key out there that you can change from it's existing value to "NTServer" I believe it was and it's all it takes to con a server into thinking it is a standalone. Then you can run NTDSutils, IIRC, to run a cleanup and then dcpromo the box again..... Sorry, but it's all a bit fuzzy now but it worked for me in a situation that appears similar to what you may have now..... I tried searching for it for you yeserday and came up with nothing except the link I put there for you. The search terms I used were:-

NTServer registry key
AD domain force demotion
demote AD NTServer registry
force demotion

and a couple of others I don't remember..... If you have a specific error number search on that.... Good luck