We put Exchange 2000 Server on a domain controller (DC). Now we want to move the DC role to a different machine. What's the least painful way to do this?

You can run Dcpromo on an existing Windows Server 2003 or Windows 2000 DC that runs Exchange; doing so will convert the machine to an ordinary member server. However, the procedure requires a reboot, and clients will lose access during that process. Microsoft doesn't support changing the role of a server by promoting or demoting it to a DC, so be careful and make good backups before you use this method. (See the Microsoft article "Overview of operating system and Active Directory requirements for Exchange Server 2003" at http://support.microsoft.com/?kbid=822179 for details.) In addition, running Dcpromo on an Exchange Server 2003 system will break Outlook Mobile Access (OMA); the only way to fix it is to reinstall Exchange (which you can do by running the /disasterrecovery switch), so the method isn't a great workaround if you're using OMA.