1. A method of securely controlling execution of a computer program within a virtual machine, the virtual machine providing an execution environment for a guest operating system, the virtual machine being configured for execution by way of system level virtualization software executing on a current host platform, the current host platform comprising at least one central processing unit having access to a mass storage device, said method comprising: executing a policy enforcer outside of the virtual machine, the policy enforcer accessing policies stored by the mass storage device, the policies identifying one or more hardware platforms for which the virtual machine is authorized to execute, the policy enforcer determining whether the current host platform matches one of the hardware platforms identified by the policies; prohibiting the virtual machine from executing on the current host platform when the policies do not indicate that the virtual machine is permitted to execute on the current host platform; and permitting the virtual machine to execute on the current host platform when the policies indicate that the virtual machine is permitted to execute on the current host platform.