What video card do you have?
According to the specs
***here*** you need a Shader 2.0 compatible video card.
System Requirements
Minimum:
Windows® XP / Vista,
Intel Pentium IV® or comparable processor with 1.6 GHz,
512 MB RAM,
128 MB RAM Graphics card (Shader 2.0 compatible)
DirectX 9.0c® compatible sound card,
3 GB free hard disc space,
DVD-ROM Drive
Keyboard, Mouse
Recommended:
Windows® XP / Vista,
Intel® Core 2 Duo or comparable processor with 2,0 GHz,
1 GB RAM 256 MB RAM Graphics card (Shader 2.0 compatible)
DirectX 9.0c® compatible sound card,
3 GB free hard disc space
DVD-ROM Drive
Keyboard, Mouse
If the game has an options menu, look for video options that you can turn down -- e.g. -- turn off shadows, turn off lighting effects, turn off antialiasing, turn off anisotropic filtering, reduce resolution...