If DCPROMO fails to demote the server, try using the brute force method:

1. Go to a run line (if you don’t know what I mean PLEASE stop here and call for technical support!).
2. Open regedt32.
3. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\ProductOptions
4. Edit the ProductType value name and change the data value from LanmanNT to ServerNT, using the exact case.
5. Restart the server.
6. When you log on be sure to delete the NTDS folder at this point.

NOTE: Make sure you remove the DC from the Active Directory Sites and Services panel as well if you have multiple DCs. Why? Seeing as the dcpromo (demotion) failed the updates are not made to ADSS and will need to be completed.