The plug-and-play pcProx Sonar solves security risks and privacy concerns with unattended computers without requiring additional software or user action. This solution is a presence detector, not a badge reader. When a user steps away from the computer, the pcProx Sonar automatically locks the computer. It detects a user"s presence so the computer will not lock until the user physically steps away, avoiding awkward time-out settings. The pcProx Sonar easily attaches to the PC via a USB port and emulates a keyboard device. This operating mode allows it to send individual keystrokes and delays to be defined by the user. For example, a Windows XP equipped system would send a sequence such as Windows "GUI" + "L" to lock the computer when the user walks away. No software is deployed on the client or server. The pcProx Sonar contains configurable flash memory which holds user definable keystrokes, delays, settings, and trigger distance (2-5 ft.).