There are five roles in a Domain Controller:
<ul>
	<li>FSMO</li>
	<li>Schema</li>
	<li>Domain Naming</li>
	<li>RID</li>
	<li>PDC</li>
	<li>Infrastructure</li>
</ul>
When removing a domain controller you need to make sure you move the roles first (if that domain controller has one of those roles.  To do so:

<!--more-->

To Transfer the Domain-Specific RID Master, PDC Emulator, and Infrastructure Master FSMO Roles:
<ol>
	<li>Open the Active Directory Users and Computers snap-in from the Administrative Tools folder.</li>
	<li>If you are NOT logged onto the target domain controller, in the snap-in, right-click the icon next to Active Directory Users and Computers and press Connect to Domain Controller.</li>
	<li>Select the domain controller that will be the new role holder, the target, and press OK.</li>
	<li>Right-click the Active Directory Users and Computers icon again and press Operation Masters.</li>
	<li>Select the appropriate tab for the role you wish to transfer and press the Change button.</li>
	<li>Press OK to confirm the change.</li>
	<li>Press OK all the way out.</li>
</ol>
To Transfer the Domain Naming Master Role:
<ol>
	<li>Open the Active Directory Domains and Trusts snap-in from the Administrative Tools folder.</li>
	<li>If you are NOT logged onto the target domain controller, in the snap-in, right-click the icon next to Active Directory Domains and Trusts and press Connect to Domain Controller.</li>
	<li>Select the domain controller that will be the new role holder and press OK.</li>
	<li>Right-click the Active Directory Domains and Trusts icon again and press Operation Masters.</li>
	<li>Press the Change button.</li>
	<li>Press OK to confirm the change.</li>
	<li>Press OK all the way out.</li>
</ol>
 To Transfer the Schema Master Role:
<ol>
	<li>Register the Schmmgmt.dll library by pressing Start &gt; RUN and typing:
regsvr32 schmmgmt.dll1.Press OK. You should receive a success confirmation.</li>
	<li>From the Run command open an MMC Console by typing MMC.</li>
	<li>On the Console menu, press Add/Remove Snap-in.</li>
	<li>Press Add. Select Active Directory Schema.</li>
	<li>Press Add and press Close. Press OK.</li>
	<li>If you are NOT logged onto the target domain controller, in the snap-in, right-click the Active Directory Schema icon in the Console Root and press Change Domain Controller.</li>
	<li>Press Specify .... and type the name of the new role holder. Press OK.</li>
	<li>Right-click right-click the Active Directory Schema icon again and press Operation Masters.</li>
	<li>Press the Change button.</li>
	<li>Press OK all the way out.</li>
</ol>
This information can be found in more detail <a href="http://www.petri.co.il/transferring_fsmo_roles.htm">here</a>.