Using Control Panel, Performance Info & Tools, Adjust Visual Effects. I have done various combinations that include unchecking Icon Drop Shadows. This works initially. But they always return on the next boot and the Icon Drop Shadows box is again checked.
The only way they don't return is to choose Max Performance. But then you have to give up every other visual enhancement, including thumbnails for photos.
It seems like a bug if a setting won't hold. I know this was discussed in another thread, but there was no solution to the problem as I describe it here. In that old thread, there were just instructions on finding and unchecking the drop shadows box.