thanks for the replies, but perhaps I didn't make myself clear enough...
I don't want to be in a permanent greyscale world, so all apps and graphics are greyscale.. I just want everything on the desktop to be greyscale - wallpaper, icons and shortcuts on the desktop, all grey, just like it looks when you're about to shut down. Any running apps I want in their full, glorious colours.
My nvidia control panel doesn't have a saturation control, apart from one for tv/video which isn't relevant.
Finally, the why... because I've small images scattered over my desktop thanks to "John's background Switcher" and I've noticed I like the desktop when its all desaturated.
goto the display settings (ati- catalyst control center\ nvidia- nvidia control panel) and goto the color settings, and in the ati panel there is an option to change the saturation, which will make it grayscale if you put it all the way to the left. I can't remember what all is in the nvidia control panel, because when I was using a nvidia card I almost never used the control panel
But I too am curious as to why you want to do this.