>I have a list box with several items in it. How does one determine if the user has clicked or control-clicked on one of the items.

You can use Windows API function GetKeyState to determine status of Ctrl key

* Click event
DECLARE SHORT GetKeyState IN WIN32API INTEGER
*#define VK_SHIFT    0x10
#define VK_CONTROL  0x11
IF GetKeyState( VK_CONTROL) < 0
  * Ctrl+Click
ENDIF