A good portion of the DVDs that are sold come with software on them to let you watch it on a PC. One of the most common ones is the InterActual player. It is owned by Sonic/Roxio, and has several versions. The latest version was just released:
InterActual 2.6 FREE Upgrade
* Cool new look!
* Great new features
* Even easier to use
* Expanded support for bonus content
You can download the latest version from the InterActual site.