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.

I see this is a Windows 7 question, but my search brought me here, so in case people in the future want to know for Windows 10... here's one possibility.

I opened Settings and searched for 'grey' and it showed where you can toggle this off and on. Below is the article from Microsoft support.

Use color filters in Windows 10

  ^{Applies to: Windows 10}

If it’s hard to see what’s on the screen, apply a color filter.  Color filters change the color palette on the screen and can help you distinguish between things that differ only by color.

To turn color filters on and off from the keyboard, press Windows logo key+Ctrl+C.

To change your color filter, select “Start” > “Settings” > “Ease of Access” > “Color & high contrast”.  Under “Choose a filter”, select a color filter from the menu.  Try each filter to see which one suits you best."