Refer to this thread regarding using TelePresence camera as a computer USB camera: using-codec-camera-webcam. In short, it's possible, but might require more work than one would like and it would be simpler to just purchase a dedicated web camera.
If you're talking about the light that is on top of the camera that is either green (idle) or orange (in call), this cannot be disabled.
You can set the camera to default to the last position it was in before the SX20 lost power, since the camera is powered by the codec. Refer to "Standby BootAction" in the SX20 TC7.3 Admin Guide on pg 102.