Is there a way to enable dithering for Windows desktop at 16bpp?

[eL_PuSHeR]

Hi. I am doing some benchmarking and I am currently running Windows XP at 16bpp instead of 32bpp (true colour). Certain programs have the ability to do dithering resulting in no noticeable quality loss, but this is not the case for common desktop windows. Is there any way to force dithering?

PS - Using an Ati Radeon HD3870 (Asus).