MS introduced built-in DVD playback capabilities with certain Vista versions and (later) Windows 7 (Starter and Basic Ed. excluded).
With Windows 8, MS plans to remove these decoders again and sell them separately as part of a “Media Center Package”. MS believes, the demand for MPEG2 playback (especially DVD) is shrinking in favour of online media (Youtube, MPEG4, HTML5), and removing MPEG2 decoders by default would give benefits on the cost side.
So, users who need (or want) MPEG2 playback will have to purchase either MS Media Center software or similar products from Cyberlink, Corel etc.