Most Helpful Reply in this webpage: http://answers.microsoft.com/en-us/windows/forum/windows_7-performance/drag-and-drop-not-working-in-windows-7/f6dcfa11-8c51-e011-8dfc-68b599b31bf5
In Windows Explorer, click a file with the left mouse button and keep the button held down. Then press the Escape key. Drag and drop is miraculously re-enabled.
I got this from http://it.thelibrarie.com/weblog/2010/06/drag-and-drop-stops-working/