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Virtual Server 2005 (VS2005) allows a server to run multiple operating systems or multiple copies of the same operating system at a single time. On servers running Windows Server 2003 and Virtual Server 2005, you can install multiple domain controllers in separate virtual machines. For many organizations, the only reason to use two separate servers to perform domain controller functions is to provide redundancy in the event of the failure of one of the two servers.
Microsoft doesn't necessarily recommend running these critical functions on virtual servers in a production environment. However, doing so can help your organization save money on server hardware by better using its existing resources.
But this solution does involve some drawbacks that you should consider. For example, when used improperly, this solution could mean risking forest-wide Active Directory corruption.
In addition, you can expect a domain controller running inside a VS2005-hosted virtual server to take a 30 to 50 percent performance hit. This occurs in part because these machines can't use multiple host processors. From a security perspective, anyone who gains access to the virtual hard disk image file could potentially misuse the information.
For more information, check out Microsoft's "Running Domain Controllers in Virtual Server 2005."