What do you mean by 'cast/stream one program/window to a raspberry pi'? What is the program/window doing/displaying - text data, graphics, video? Do you have access to the source code of the program whose output you are trying to send to the pi? Do you just want to display the Windows program's screen output on the pi's display, or do you want to use it as input to a program running on the pi?
AFAIK, there is no easy way (at the Windows OS level) of displaying a single Windows task window on another computer system. If you describe more precisely what you want to do, someone may be able to suggest a way of doing it.