A computer is adapted for pay-for-use operation by adding a isolated computing environment to a standard computer. The isolated computing environment may include a trusted non-volatile memory, a digital signature verification capability, a clock or timer and a logic circuit for triggering execution of a validation program responsive to the clock or timer. The isolated computing environment may be protected from tampering by physical or cryptographic mechanisms, or both. The validation program measures or monitors for non-compliant states of the computer and may enforce sanctions when non-compliant states of the computer are detected.