Use the Windows API function ExtractIcon(), passing it the instance handle of your application, the path name of the application you wish to extract the icon from, and the number of the icon you wish to extract. Example:

var
  TheIcon: TIcon;
begin
  TheIcon := TIcon.Create;
  TheIcon.Handle := ExtractIcon(hInstance,
                                'C:\SOMEPATH\SOMEPROG.EXE',
                                0);
 {Do something with the icon}
  TheIcon.Free;
end;