var theWidth = 500;
var theHeight = 640;
function popWin(url){
    var theTop = (screen.height/2) - (theHeight/2);
    var theLeft = (screen.width/2) - (theWidth/2);
    var features = 'height='+theHeight+',width='+theWidth+',top='+theTop+',left='+theLeft+",scrollbars=yes";
    theWin=window.open(url,'',features);
}