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Download it again 

Where to download the various versions of DirectX

DirectX 11

•  The latest operating system from Microsoft already includes DirectX 11. There is no need to download any specialised updates. However, Microsoft may issue tweaks or security updates for DirectX 11 as part of its monthly patch releases. Windows 7:
  
  


• Windows Vista: Originally Windows Vista shipped with DirectX 10 but you can download a separate DirectX 11 update. Firstly, you must have Windows Vista Service Pack 2 installed (you can get a copy by following the download link above). Now here is a trick from Microsoft - the update was released as part of the Platform Updates package (which is why it may not be immediately obvious that it includes the DirectX 11 update). To download and update DirectX 10 to DirectX 11, do the following:
  
  
  1. Click Start - All Programs - Windows Update.
  
  
  2. In the navigation pane, click Check for updates.
  
  
  3. When updates are found, click View available updates (updates are not visible to download if you already have them on your system).
  
  
  4. Select the Platform Updates package, and then click Install.
  
  


• Windows XP, Windows 2000 and earlier: DirectX 11 and DirectX 10 are not compatible with these operating systems. Some unofficial hacks are available, but there are questions about their degree of suitability for the average PC user. 
  
  DirectX 10
  This release has become an orphan. It will only run on Windows Vista and even Vista can be updated to DirectX 11. For running DirectX 10 on Windows XP or 2000, see DirectX 11 above.
  
  DirectX 9.0c
  DirectX 9.0c is the last officially supported version for Windows XP and 2000. It remains an important part of Windows Vista and Windows 7 - which is why both of these newer operating systems have it installed. 
  
  DirectX 8.1
  Version 8.1 is the last available version for the ancient operating systems of Windows Me and Windows 98.