pixel shader for full screen?

Hi
As far as I know, pixel shader lets me to modify texels drawn in the 
texture on vertex. But how should I made it, when I wish to make a 
shader program to make effect for full screen. Let's say I wish to make 
screen black & white or change visible resolution or other effect for 
full screen. I wish to normally draw my 3D scene and make it looks 
different using some effect.

Thanks
G