var myWindow;

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