Abstract: A security system for a computer (10) having at least one communications port (12) for connection to peripheral devices, includes a card reader (14). A switching means (22) connected to the card reader and connectible to the communications port, is operable to connect the card reader to the communications port in response to a command including protected routines which generate said command to cause the switching means to connect the card reader to the communications port. Suitable software compares data from a card read by the card reader with authorisation data held in the computer, to enable or disable further operation of the protected routine(s).