Cannot unselect all in Excel

When in excel, I click once in a cell and it automatically selects all cells. Wherever I move my mouse the cells are selected. I cannot close the window or chose anything on the menu bar. The only way to close the window is to Control-Alt-Delete and end the task. It first occured in Office 97, but I uninstalled it and installed Office 2000, and it still occurs! Nothing appears to be wrong hardware-wise. Could there be something in the Registry that just won't go away? Any help would be greatly appreciated.