Turn Windows 7 desktop into grayscale

The NVIDIA drivers allow you to do this. Go into the NVIDIA Control Panel (by right-clicking on the desktop, then selecting "NVIDIA Control Panel" from the context menu OR by running it from Control Panel, where it'll be under "Hardware and Sound" when using the category view).

Find the "Display" category, then "Adjust desktop color settings". The main panel will then allow you to choose a monitor and at the bottom there is section "3. Apply the following enhancements:" The first option there is "Digital vibrance", which basically sets the saturation of the display. The default value is +50%. By setting it to 0, all colours are desaturated and end up being greyscale. Repeat for multiple monitors.