A computer security system which prevents an unauthorized user from accessing the computer system when an authorized user has already logged onto the computer system and has temporarily left the workstation. The computer security system generally includes a sensor which is configured to detect the presence of a person in the region around a workstation and a processing unit which logs out of the computer when a person is no longer present in the region. An optional calibration tool may be provided to set the sensitivity of the sensor and prevent accidental logging out when the authorized user is still present in the region around the workstation.