No matter the legality or format of your acquired copy, you need a media player capable of playing DVD movies. Some players can't play them at all. Some can play them but with limited functionality. What you generally want anytime you want to play a DVD on a computer, is one that is designed for DVDs. There are several of these, I use "Zoom player" now and then if I'm desperate enough to use the computer rather than TV to watch. Assuming you have a disc, all you need to do is insert it and open the disc drive from the player. If you don't have a disc, well, that's your problem.