You'll need to capture and stream the video from the Pi to your desktop. There's an example of streaming mjpeg video from a device using Python/Flask, to a second device running Python/OpenCV here on chioka.in. The example is a little too long to post in full, and may require some tweaking to get it working with a USB camera on the Pi. This should work fine if it's run when the Pi boots, meaning that your camera feed starts when the Pi does and stops when it's shut down.
If you would like to be able to trigger starting and stopping the camera you'd be best off doing some further research on Flask and implementing suitable controls in the index.html file.