function popUpWindow(aURL, aWidth, aHeight, aScroll) {

  var zLeft = (screen.width - aWidth) / 2;

  var zTop = (screen.height - aHeight) / 2;

  var zWinProp = 'width='+aWidth+',height='+aHeight

               + ',left='+zLeft+',top='+zTop+',scrollbars='+aScroll+',resizable';

  zWin = window.open(aURL, '', zWinProp);
  zWin.window.focus();
}