If you are a game player or heavy multimedia user, you need to make sure you are running the current version of DirectX which is the multimedia system foundation for Microsoft Windows operating systems. See DirectX Home User Download page. From the site:

For Windows NT users with service pack 3 or higher:
You have DirectX 3.0a ("DirectX Foundation") functionality. Users of applications such as Windows Media™ Encoder on NT 4.0 will require DirectX Media 6, the highest release supported on this platform.

For Win9x, ME, Windows 2000 user:
DirectX 8.0a - this version of DirectX offers updated graphics, faster frame rates, and support for massively multiplayer games. It also offers more immersive audio when running and displaying programs rich in multimedia elements such as full-color graphics, video, 3-D animation, and surround sound.

The point is, if you want best media performance, make sure your drivers are current including support for DirectX. At the time of writing, DirectX 9 is in beta. These tips age. Go to the DirectX home page and check to verify you are running latest.