Manual Removing A Domain Controller

I am running Windows 2000 Server. I have two Domain Controllers. I setup another server as a Domain Controller. I found I did not want this server (running Exchange 5.5) as a Domain Controller. I ran dcpromo to remove it. I thought it was removed but it was not removed. It still thinks the server is a DC but it will not replicate with other DC’s and gets DNS Lookup errors and replications errors. I can not add or remove the server as a DC. How can I manually make this system a stand along server again?? Can I go into the register and removed the items that were not removed. The server acts as an Exchange Lotus Notes connector and I want to upgrade to Exchange 2000 but I know I will have problems. Thank you..