Have you used Windows before? Yes, you can as many programs on your computer as your resources will allow. Of course, the programs are not all running at the same time really you just get the impression that they are.
No. That's not what I meant. Say for instance I had a program that took input. Well, when you put in a certain input; it would open up photo viewer and show a certain picture. Is there a way to do that? I wanted to use the default windows image viewer. Or would it be easier to just make my own photo viewer part of the program?