From this Microsoft article we read:
After upgrading the NT4 domain to Windows 2000/2003 Domain, you may find out that the old BDC (Backup Domain Controller) computer account still exits in the Active Directory.We have the following options to remove the old BDC from the Active Directory:
- Option A: Upgrade the current BDC server to Windows 2000/2003.
- Option B: Remove the computer account from the Active Directory from the BDC.
1. Log on into the BDC with an account that is a member in "Domain Admins" users group.
2. Go to "Start" -> "Run" -> Write "srvmgr" and press on "Enter" button.
3. Remove the BDC computer account from the domain.
- Option C: Remove the computer account from the Active Directory manually.
1. Logon into a Windows XP workstation or into the domain controller with an account that is a member of "Domain Admins" users group.
2. Install Windows Support tools.
3. Go to "Start" -> "Run" -> Write "adsiedit.msc" and press on "Enter" button.
4. If your FQDN domain name is: domain.local and the BDC remains in "Domain Controller" you can use the following instructions:
a. Expand Domain NC, expand DC=Domain,DC=Local, and then expand OU=Domain Controllers.
b. Right-click the computer name BDC, and then right click and choose "Delete"..
Note: Removing the computer account will prevent the BDC functions and will drop the option to use network resources from the domain.