On my PC, if you check my specs it is pretty good. I can run any DX9 game with no trouble, I didn't even have to install DirectX 9. I have Sims 2, and when I got my PC, straight on goes the Sims, and by the way, it was a clean install.
I thought that Windows 7 has it inbuilt, or maybe it is because I have COD4, I don't know, but you shouldn't have to re-install DX9.