It's under "Adjust ClearType text" on the control panel in Windows 7. It's also known as "ClearType Text Tuner."
It doesn't specifically ask you to pick your subpixel layout, but rather shows you a bunch of different renderings and asks you to pick the one you think looks best.
It doesn't specifically ask you to pick your subpixel layout, but rather shows you a bunch of different renderings and asks you to pick the one you think looks best.