Win-NT 4 File Copy Anomaly

My shiny-new NT-4 Workstation installation used to copy/move files by the drag-and-drop method, but all of a sudden it quit doing that. I get a “no-go” symbol whenever I try to move/copy a file. However, the Edit | Copy function behind Explorer’s Edit menu still works fine. How can I restore the drag-and-drop function?
-Bob Hanlon-