Microsoft has been talking about this for quite a long time; D3D10 (the actual name, notice it's not DX10, there is no DX10 any longer) is available only for Vista because it is directly tied to the new Windows Display Driver Model (WDDM). Here is a Microsoft Powerpoint named "Intro to Direct3D10" that explicitly defines D3D10 being available only on the Vista platform (see pg 68) There are other sites out there too, just hit Google and type in D3D10 and you'll find an onslaught. If you do Google, make sure to read the search results carefully -- you can install the SDK for development of code, but you can't actually run a D3D10 renderer on anything other than a Vista OS. Sorry to give everyone the downer, I figured most people knew this by now if they were following Microsoft's DirectX announcements with any frequency