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?
Heading out to the gardens!