motion detection program for pc (to monitor a window or an area on a window)

Hi, at my work we have a cheap security camera system (manufactured by Lorex) and its hooked up to our router and I would like my pc to make an audio tone if someone drives into the parking lot. It is ok if it is not 100% accurate and gives a false alarm every once and a while, because I would not be using it every day, I would only be using it when I am expecting someone after hours (so I can set it and when they pull in I will hear it beep so I can go unlock the door). In my work area I can not hear the door if they knock, so an audio tone would be great if I can figure out a program to do that.

Below is a screen shot of what the web login looks like.

The cam with the red box has a pretty good overview of the lot, so if I could find some program to monitor the colors and if a major change is noted it would beep. I can maybe put in the xy cordanates that I want to monitor if it is an option.

Or I can make that cam (circled in red) take the full window for better monitoring posably. Does anyone know of such a program?