A. The simplest way to install DirectX 9.0a is to navigate to the Windows Update Web site at http://windowsupdate.microsoft.com, click "Pick updates to install" from the left-hand pane, select the "DirectX 9.0a End-User Runtime" link from under the XP or Win2K section, then click Add and proceed with Windows Update as usual.
Alternatively, you can visit the DirectX Web site at http://www.microsoft.com/directx and click Downloads from the left-hand site navigation menu. Select the "Get DirectX 9.0a Now" option, then download dxwebsetup.exe. After you download the file, run the file to download the files required for installation.