Hacking a rental computer to use it beyond purchased rental time is prevented by plugging a time card with internal counter into a DIMM socket of the motherboard and encrypting the register locations of the time card with the private key of the motherboard to bind the time card to the motherboard. Thus, if the time card is not detected at boot or if it is removed during operation the computer is disabled. The counter counts down the rented time period as it receives clocking signals, and at the elapse of the purchased period disables the computer.