DirectX Diagnostic tool dxdiag is a useful tool on Windows that gives you loads of information about your system, such as GPU, CPU and BIOS info. It is common that tech support asks you to attach your dxdiag report to support requests since it helps to narrow down the problem.
Before Windows 8, dxdiag was installed whenever you installed any version of DirectX - in Windows 8, it is installed right on with the operating system.
To open the Diagnostic tool, press the Windows button to open the main menu and type run. In the Run prompt, enter dxdiag, hit Enter and the tool will open. You can now browse through the tabs as well as save a report on your system as text file by clicking on the Save All Information... button. This report is what is requested by tech support when you are asked to 'attach the dxdiag output'.