iTunes Movies Problems Playing on Other Computers

I've downloaded 300+ of my iTunes movie library, yet it won't let me watch the movies on a computer other than my own. I tried watching a movie on my girlfriends laptop and it just shows a black screen. We logged into her iTunes account on her computer, and it still won't let us watch the movie. I hook the hard drive back up to my computer and it starts playing immediately. These movies are downloaded and saved to a portable hard drive. I've paid for them, I own them. Why will it not work on her computer?