basta modificarlo leggermente
codice:
<script type="text/javascript">
function open_win()
{
window.open("http://www.sito.it/show.php?id=<? echo $id ?>","_blank","toolbar=yes, location=yes, directories=no, status=no, menubar=yes, scrollbars=yes, resizable=no, copyhistory=yes, width=400, height=400")
}
</script>