var myWindow;
function openCenteredWindow(url, width, height)
{
  var left = parseInt((screen.availWidth/2) - (width/2));
  var top = parseInt((screen.availHeight/2) - (height/2));
  var windowFeatures = "width=" + width + ",height=" + height + ",status,resizable,left=" +
                       left + ",top=" + top + ",screenX=" + left + ",screenY=" + top;

  if(myWindow && !myWindow.closed)
    myWindow.focus();                               
  myWindow = window.open(url, "subWind", windowFeatures);
}