Renew your computer or buy stuff for it. My I built my computer from different parts and it runs game greatly. It proabably cost me a $1000 or 2. I don't know, I forgot.
More than likely you need a video card with newer pixel shader support (2.1 or better maybe.) I had this problem with Thief 3. With my MX440 I couldn't run it, so I had to buy a new card.
