Microsoft
At the core of DirectX are its application programming interfaces, or APIs. The APIs act as a kind of bridge for the hardware and the software to "talk" to each other. The DirectX APIs give multimedia applications access to the advanced features of high-performance hardware such as three-dimensional (3-D) graphics acceleration chips and sound cards. They control low-level functions, including two-dimensional (2-D) graphics acceleration; support for input devices such as joysticks, keyboards, and mice; and control of sound mixing and sound output. Because of DirectX, what you experience with your computer is better 3-D graphics and immersive music and audio effects.
DirectX Redistributable 9.0c November 2008
http://download.microsoft.com/download/1/7/1/1718ccc4-6315-4d8e-9543-8e28a4e18c4c/dxwebsetup.exe
http://download.microsoft.com/download/5/8/2/58223f79-689d-47ae-bdd0-056116ee8d16/DXSDK_Nov08.exe