Does it give you any errors? Does it install then give you an error, or
just not able to install at all? We need a little more information:
computer specs, error messages, etc..
Let us know and we should be able to get you up and running.
One tip, though: make sure your video card drivers are up to date.
Doom 3 ran just fine on my system. It was the first game I installed on my
new PC. I did run the official id Software patch for it though. I didn't
have to install DX9 on my system either.