Did you try dcpromo /forceremoval on the faulty server ?
I would suggest to move the topic on Directory Services there;
http://social.technet.microsoft.com/Forums/en-US/winserverDS/threads, to get more answer.
I would check too in AD site and service to be sure the DC is no longer there too from your DC that still work. To prevent sync error to a server that is no longer a DC.
http://support.microsoft.com/kb/258062 - "Directory Services cannot start" error message when you start your Windows-based or SBS-based domain controller
MCP | MCTS 70-236: Exchange Server 2007, Configuring