Minimum System Requirements
OS: Windows XP SP2/Vista
Processor: Intel Core 2 DUO E4600 (2.4 GHz)/AMD Athlon64 X2 5200+
Memory: 1.5 GB
Hard Drive: 1.5 GB Free
Video Memory: 512 MB (NVIDIA GeForce 7800/ATI Radeon X1900)
Sound Card: DirectX Compatible
DirectX: 9.0c
Keyboard & Mouse

Recommended System Requirements
OS: Windows Vista
Processor: Intel Core 2 QUAD Q6600 (2.4 GHz)/AMD Phenom X4 9500 (2.2 GHz)
Memory: 2 GB
Hard Drive: 1.5 GB Free
Video Memory: 512 MB (NVIDIA GeForce 9800 GTX+/ATI Radeon HD 3850)
Sound Card: DirectX Compatible
DirectX: 10
Keyboard & Mouse

Developer comment

The game looks slightly prettier with directx10 as you can have more advanced shaders. I didn't set the recommended specs, but I suspect that's why the recommended is Vista. I can assure you though, it runs perfectly fine on XP as well.