Pixel shaders are a programmable part of graphics pipeline that execute on the graphics processing unit (GPU) on a graphics card. They offer tremendous capacity for rendering realistic texture, shading and other effects. At the same time this offloads graphics computations from the central processing unit (CPU). Previously pixel shaders were primarily used only in games and very high-end graphics systems; with Windows Presentation Foundation pixel shaders are used to render applications on the desktop.
Windows Vista (as well as WPF) gives high performance if you are using Pixel Shader card.