These are the minimum and recommended system requirements for the game. It could be that your video card or processor aren't up to the system requirements.
Minimum system requirements: Windows XP, Vista, 7, 8, processor Intel Pentium IV 2.8 GHz or equivalent processor with SSE3 (Intel Core 2 Duo 2.6 GHz or equivalent recommended), 2 GB RAM, NVidia GeForce 8600 GT / ATI Radeon HD X1800 video card with 256 MB VRAM and Shader Model 3.0 support (NVidia GeForce 9800 GT / ATI Radeon HD 4850 or better recommended), 3.5 GB HDD, DirectX 9.0c-compatible soundcard.
If your computer isn't up to running the game, are there any settings you can turn down in the game options?
In my PC corner for the winter, gaming, knitting and reading.