Rod Trent
posted over on his blog some thoughts on running a domain controller as a VM (virtual machine). I can think of two places I would do this. Disaster recovery and as part of a "swing" upgrade to active directory from an NT4 domain. In most instances you are building a new destination DC on new hardware, and have an NT4 domain controller on older hardware. Build the new Windows 2003 Server and then build an NT4 server on it as a VM. Promote it to be the PDC. Upgrade it to be the new AD domain’s DC. The other good thing about a VM is you don't need to worry about device drivers for NT4 on new hardware which can be especially tricky now that it is "retired."