function centerWindow(url)
{
    if (document.all)
        var xMax = screen.width, yMax = screen.height;
    else
        if (document.layers)
            var xMax = window.outerWidth, yMax = window.outerHeight;
        else
            var xMax=640, yMax=480;

    var xOffset = (xMax - 700)/2, yOffset = (yMax - 450)/2;

    window.open(url,'Help','toolbar=0,location=0,directories=0,status=1,menubar=0,scrollbars=1,resizable=1,width=520,height=450,screenX='+xOffset+',screenY='+yOffset+',top='+yOffset+',left='+xOffset+'');
}