Does your computer fit all of these and are you using a graphics card from the list that is not supported?
Operating System: Windows XP or Windows Vista
CPU: Intel® Pentium™ III 1000 MHz or AMD® Athlon™ 1000 MHz
RAM: 512 MB or more
Video: 64 MB 3D graphics card with Hardware Transform and Lighting, such as NVIDIA® GeForce™ 3 class card or newer, ATI® Radeon™ 9700 or newer.
DirectX® 9.0c and latest video drivers
Exceptions:
The following cards are not supported:
Geforce™ FX Series
Geforce4™ MX Series
GeForce™ PCX 5900
GeForce™ 6200 Series
Intel® Graphics Media Accelerator X3000
ATI® Radeon™ X300 Series
ATI® Radeon™ X1300 LE HyperMemory-32
Sound: Card capable of supporting DirectX 9.0
Hard Drive: 4.0 GB available space
Internet Connection: 56k or better