Microsoft has posted a knowledge base article on how to enable the DVD Library feature on a Windows Vista based Media Center. This feature is enabled by default in Microsoft Windows XP Media Center Edition 2005. The DVD content is stored on the hard disk, and appears in the My Videos folder.
On Windows Vista-based Media Centers content is displayed in a DVD gallery. However, the DVD Library is not automatically enabled in Windows Media Center on a Windows Vista-based computer. This feature should allow you to play your commercial DVDs from the hard drive.