You cannot simply remove a domain controller from a domain. To do so, you must run DCPROMO to remove Active Directory. Prior to doing so, it is very important that all FSMO Roles held by the domain controller being removed from AD are transferred to another domain controller, if applicable. Please refer to the following MSKB article http://support.microsoft.com/kb/324801.
Prior to demoting the server, make sure that you have a good backup of the system state so that you can perform a non-authoritative restore of AD once the server is rebuilt. This is important because if you simply rebuild the server with the same name and promote it to a domain controller, then allow it to replicate, there is a distinct possibility that AD will become polluted and you will start to see issues arise as a result of the residue in AD left by the original domain controller. Yes, even though you perform DCPROMO, some information pertaining to the server may not be removed from AD. You really need to watch out for this if you cannot remove the server cleanly and have to force its removal from the domain.
If you do plan to rebuild the server and not perform a non-authoritative restore of AD — that is simply allow the DC to replicate from another DC, then you will need to perform some additional steps using the NTDS Utility beforehand. Please see the following MSKB article http://support.microsoft.com/kb/216498,