You are having problems promoting or demoting a domain controller and you want to troubleshoot it.
The best source of information about the status of promotion or demotion problems is the Dcpromo.log and Dcpromoui.log files contained in the %SystemRoot%\Debug folder on the server. The Dcpromo.log captures the input entered during dcpromo and logs the information that is displayed as dcpromo progresses. The Dcpromoui.log file is much more detailed and captures discrete actions that occur during dcpromo processing, including any user input.
Additionally, the Windows Server 2003 version of dcdiag contains two new tests that can aid in troubleshooting promotion problems. The dcpromo test reports anything it finds that could impede the promotion process. The RegisterInDNS test checks if the server can register records in DNS. Here is an example of running both commands to test against the rallencorp.com domain:
> dcdiag /test:dcpromo /DnsDomain:rallencorp.com /ReplicaDC /test:RegisterInDNS
In most cases, the level of detail provided by Dcpromoui.log should be sufficient to pinpoint any problems, but you can increase logging if necessary. To enable the highest level of logging available, set the following registry value to FF0003: HKLM\Software\Microsoft\Windows\CurrentVersion\AdminDebug. You can confirm that this mask took effect by running dcpromo again, checking the Dcpromoui.log, and searching for "logging mask." For more information on the various logging settings, see MS KB 221254.
If you get desperate, the Network Monitor (netmon) program is very handy for getting a detailed understanding of the network traffic that is being generated and any errors that are being returned. You can identify what other servers it is talking to or if it is timing out when attempting to perform certain queries or updates.
MS KB 221254 (Registry Settings for Event Detail in the Dcpromoui.log File), and MS KB 260371 (Troubleshooting Common Active Directory Setup Issues in Windows 2000)