Shader Lightining Problem

[Windfall]

I'm having a weird lightening problem which makes the game unplayable. If the shader is set between low - high, the game looks realistic & dark but the framerate drops unbelievably, 1 - 2 fps! if the shader is set to 'very low', it results in huge framerate increase but also turns the game so bright that it becomes unplayable. Main problem is, the lightening seems to be connected with shader. Have a look:

Low shader: Too dark


Medium shader: Little less dark


High shader: OK


Very low shader: Too bright & FPS increase.


My video card is ATI Radeon Xpress 200 (equivalent to X300, only integrated in motherboard)
3D Graphics:
Integrated Radeonâ„¢ X300 based
Full DirectX 9.0 Support (Vertex Shader v2.0 and Pixel Shader v2.0)
Supports resolutions up to 2048x1536@32bpp
SmartShaderâ„¢ 2.0 allows complex, movie-like 3D effects
SmoothVisionâ„¢ 2.1 improves image quality through anti-aliasing using multi-sampling algorithm with support for 2,4, and 6 samples
Graphics Memory: 256 MB

Catalyst Control Center Version: 7.2 (A.I disabled as said in game manual)
Others: Processor: Intel Pentium D 2.8GHz, OS: Windows XP MCE 2005, RAM: 1.5 GB

So, what's the deal?