Pixel Shaders is a technology which is being used by most of the latest games which makes use of realistic surface light reflection. For example:... when playing Prince of Persia Warrior Within, and you look at a particular wall which is being hit by a source of light, the light will realisticall be projected on that wall with a nice effect that shows the grooves and bumps of the walls in detail.
So this is practically only a graphical feature which only the latest graphics cards support. Unfortunately, the GeForce 4 MX does not support the Pixel Shaders used in PoP:WW, so the best thing to do is to buy a new graphics card. I would recommend something in the line of a ATi Radeon 9800PRO which runs excellent on PoP:WW, and the Pixel Shaders are seen too. There 3 types of pixel shaders technologies, each on detailed more than the other, which are Pixel Shaders, Pixel Shaders 2.0 and Pixel Shaders 3.0 and I think PoP:WW uses Pixel Shaders 3.0.
Hope this helps.