You don't "download" pixel shaders, lol.
Pixel Shading is a technology that is literally built into the very graphics chip itself. You can't get a better pixel shader without buying a new GPU unless you're already at the top.
Windows Key + R, type in "DxDiag" without the quotes. Go into display tab, tell me what your video card manufacturer is. It should be either ATI, Nvidia or Intel.