A method for preventing unauthorized modifications to a rental computer system is disclosed. During boot up of the rental computer system, a determination is made whether or not a time-day card is bound to the rental computer system. If the time-day card is bound to the rental computer system, another determination is made whether or not a time/date value on the time-day card is less than a secure time/date value stored in a secure storage location during the most recent power down. If the time/date value on the time-day card is not less than the secure time/date value, yet another determination is made whether or not the time/date value is less than an end time/date rental value. If the time/date value is less than the end time/date rental value, the rental computer system continues to boot.