Check the Kinect cable and make sure that it's attached firmly to the console.
Reposition the Kinect sensor to give it more space. See Troubleshoot body tracking for more information.
Walk through the Kinect initial setup again. See Kinect setup.
Recalibrate the Kinect sensor. See Calibrate the Xbox 360 Kinect sensor.
Would you like to contact Support regarding the "Error C005120B occurs when using your Xbox 360 Kinect sensor" issue you were reading about or a different issue?