If you have implemented VMware’s ESX on your server as the core virtualization technology, a license is required for every instance running in the virtual environment. If you have only assigned one license to Windows Server 2008 Standard while using ESX, you may only run one instance of the software at a time. Windows Server 2008 cannot be deployed as the operating system in the physical environment since ESX takes its place.
If you have the Enterprise version – you are permitted to run four instances at a time only, taking into account that the normal circumstances of being able to run a fifth instance on hardware virtualization software and software managing and servicing the OSEs on the server does not apply.