"The DirectX Diagnostic Tool is designed to help you troubleshoot DirectX-related issues.
The default installation folder for Dxdiag.exe for Microsoft Windows 2000 and Microsoft Windows XP is:
C:\Windows\System32
To start the DirectX Diagnostic Tool, click Start, click Run, type dxdiag in the Open box, and then click OK.
The following list contains a description of each tab in the DirectX Diagnostic Tool dialog box.
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System
The System tab provides system information about your computer and specifies the version of DirectX that is installed on your computer."
source
http://support.microsoft.com/kb/190900