Monitors usually have OSD (on screen display) settings that allow you to desaturate its output. That's the easiest method.
Catalyst Control Center (for ATi/AMD graphics) has plenty of options for the same. Just explore and you'll where the colour controls are. No idea for nVidia graphics since I don't have a modern nVidia graphics card.
Windows does have a colour management option for display drivers, although I don't know how to tweak it for a monochrome effect.