var mywindow;
  var features='width=770,height=435,left=200,top=200,scrollbars=1'

function init(){
document.getElementById('purchase').onclick=function() {
   popUp(this.href+'?'+document.getElementsByTagName('img')[0].title)
   return false;
    }
   }
function popUp(url) {
if(mywindow) {
   mywindow.close();
 }
    mywindow=window.open(url,'myname',features);
    mywindow.focus();
 }

if(window.addEventListener){
   window.addEventListener('load',init,false);
 }
else { 
if(window.attachEvent){
   window.attachEvent('onload',init);
  }
 }