Scenario:

Parent domain dies, leaving a poor little orphan domain.
Little orphan domain now wants to be independent or join another family (domain) so it tries to run DCpromo.

Dcpromo says: "Unless I can contact your parent I cannot set you free"
Little orphan replies: "But my parent is dead!"

Seriously, how can you remove ADS from a server in domain that has lost it’s parent domain, when DCpromo can no longer contact the now non-existing parent domain anymore.?

Scenario:

Forest Root DC, also GC, holding all FSMO’s has a corrupt copy of the AD database.
You need to keep the machine and the production domain intact.!

Here is a working, non-supported, way of doing this.

1. On another DC in the domain run NTDSUTIL to move the FSMO’s, that is seize them!
2. Make sure DNS is 100% solid on the working DC.
3. Make sure another working DC is also a GC.
4. Boot the corrupt DC into Active Directory Restore Mode, and edit the following registry key:

   Hive:	HKEY_LOCAL_MACHINE
   Key:	SYSTEM\CurrentControlSet\Control\ProductOptions
   Name:	ProductType
   Type:	REG_SZ (String value)
   Value:	LanmanNT (the old value)
   Value:	ServerNT (the new value)

   Yeah, one key baby!

5. Open a command prompt and type: net stop ntfrs to stop File Replication Service.
6. Delete WINNT\SYSVOL and NTDS directories.
7. Reboot the now former DC.
8. Log into the now member server and change it to a stand alone, by joining a workgroup.
9. Reboot the now stand alone server.
10. On the good DC delete the disabled computer account for the old, now defunctional, old DC.
11. On the stand alone run DCPROMO to join the domain/tree/forest again.
12. Reboot and use the new DC.