A. Microsoft recently changed the licensing model for Windows 2003 R2 Datacenter Edition around the execution of virtual OSs running on the platform via Microsoft Virtual Server 2005 R2. You can now run an unlimited number of virtualized Windows Server instances on a Windows 2003 R2 Datacenter server. This includes virtualized Standard, Enterprise, and Datacenter instances running Windows 2003 R2 or any previous version. You can find more information here. Note that although it discusses Virtual Server 2005 R2, the unlimited virtualization usage rights apply to any virtualization technology.