var win = null;

  function closePopup()
  {
    if (win != null) win.close(); 
      win = null;           
  }

  function openWindow(wnd, _width, _height)
  {
    closePopup();
    posx = (screen.width) ?   (screen.width-_width)/2 : 0;
    posy = (screen.height) ? (screen.height-_height)/2 : 0;
    settings = 'height='+_height+',width='+_width+',top='+posy+',left='+posx+',scrollbars=yes,resizable=yes,alwaysRaised=yes'
    win = window.open(wnd,'popwindow',settings)
    if(win.window.focus){win.window.focus();}
  }