This one is for gamers using Vista as their OS of choice. While I do not use Vista yet and so far have not run into any such issue where my DX 10 (not that i have it) installation is corrupted by DX 9, this chap at TechArp is facing this issue.
In his words, the issue is:
Hope it helps if any body has encountered this issue.
Link goodie: Tech ARP - ED#83 : Installing DirectX 9 Can & Will Kill DirectX 10 Rev. 2.0
In his words, the issue is:
After analyzing what happened, I came to the conclusion 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.
If you face the same problem, try reinstalling DirectX 10. You can download the latest version here. This redistributable package does not require WGA validation to download.
Hope it helps if any body has encountered this issue.
Link goodie: Tech ARP - ED#83 : Installing DirectX 9 Can & Will Kill DirectX 10 Rev. 2.0