if your machine is running win7 you won't be able to install dx9c, because you're already running dx11, which is the latest version.
The Witcher works fine under dx11 installation as it runs in dx9 mode which dx11 is compatible with. It sounds like your problem is with the game not installing correctly. I recommend to do a system restore, delete any files you can regarding the game, then do a registry clean up and try to install the game again.
I have it running on my win7 machine with no problems.