Provalo così
codice:
<script> 
var popunder="http://www.iltuositoqui.com" 
var propr="width=100 height=200 resizable=no"
var once_per_session=1 
function get_cookie(Name) { 
var search = Name + "=" 
var returnvalue = ""; 
if (document.cookie.length > 0) { 
offset = document.cookie.indexOf(search) 
if (offset != -1) { // if cookie exists 
offset += search.length 
// set index of beginning of value 
end = document.cookie.indexOf(";", offset); 
// set index of end of cookie value 
if (end == -1) 
end = document.cookie.length; 
returnvalue=unescape(document.cookie.substring(offset, end)) 
} 
} 
return returnvalue; 
} 

function loadornot(){ 
if (get_cookie('popunder')==''){ 
loadpopunder() 
document.cookie="popunder=yes" 
} 
} 

function loadpopunder(){ 
win2=window.open(popunder,'',propr,'') 
win2.blur() 
window.focus() 
} 

if (once_per_session==0) 
loadpopunder() 
else 
loadornot() 

</script>