This Arduino UNO project causes an unattended computer in a secure environment to lock up automatically when the computer is unattended for a defined period of time, requiring password input to unlock the system. The project uses an ultrasonic transducer to sense the presence of the user within a defined distance. If that distance is not maintained for a given period of time (hysteresis period), the user is assumed to have left the computer. The USB port on the UNO is reprogrammed to make the device seem to be an HID (Human Interface Device - e.g. a USB keyboard), and sends a CTL-ALT-DEL-"K" key sequence to force the Windows XP computer to lock and require password input to unlock. In the pictures, the LCD display is merely a debug tool. A three wire connection to the ultrasonic transducer allows the transducer to be placed conveniently for a given installation. I am currently working on an enhancement consisting of an RFID tag reader which will automatically unlock the computer when the users ID badge is within range.