Lower-privileged processes cannot drag-and-drop to higher-privileged ones on Vista and later.
The bottom line is that drag and drop from Windows Explorer will not
work if your application is elevated.
http://blogs.msdn.com/b/patricka/archive/2010/01/28/q-why-doesn-t-drag-and-drop-work-when-my-application-is-running-elevated-a-mandatory-integrity-control-and-uipi.aspx
