DirectX 10 was only available for Vista and not XP. In order to make DirectX 10 compatible with Windows XP, you need to make a lot of changes to various system files responsible for creating the display on your screen (also called rendering). There are some hacks available out there to enable version 10's features on XP and they often work for particular setups - however, this does not mean they will work in all circumstances. In other words, use them at your own peril: there is no official release from Microsoft for running DirectX 10 or DirectX 11 on XP.
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