1. A method of securely controlling the execution of a software program within a virtual machine as executed on a host computer system, said method comprising the steps of: securely restricting execution of a virtual machine to an authoritatively identified host computer system; and securely restricting execution of a software program within said virtual machine to an authoritatively identified instance of said virtual machine, said software program being executable on a virtual host computer system encapsulated within said virtual machine; wherein: said securely restricting execution of the virtual machine selectively provides for the termination of execution of said virtual machine on non-authoritatively identified host computer systems, and said securely restricting execution of the software program selectively provides for the termination of execution of said software program within non-authoritatively identified virtual machines.