Quote: that the recent installation of some DirectX 9 games was the cause. Those games would install DirectX 9.0c as part of their installation process, just to ensure your PC would have the latest DirectX version installed.
By right, the reinstallation of DirectX 9.0c should not affect DirectX 10 at all. However, it appears that the reinstallation of DirectX 9.0c by those games have somehow corrupted DirectX 10 in Windows Vista. The solution was simple - reinstall DirectX 10, but this should not have been necessary. Microsoft should look into why installing DirectX 9.0c corrupts DirectX 10.