function openCenteredWindow(url, windowname, width, height) 
{
    var left = parseInt((screen.availWidth/2) - (width/2));
    var top = parseInt((screen.availHeight/2) - (height/2));
    var windowFeatures = "width=" + width + ",height=" + height + ",scrollbars=no,menu=no,status=no,resizable=0,left=" + left + ",top=" + top + "screenX=" + left + ",screenY=" + top;
    myWindow = window.open(url, windowname, windowFeatures);
}