Pop-under not working

[canaani]

I wonder if anyone can help me with this: I have
created a pop-under window which should appear when
users leave the site www.dolphinswims.co.uk
It doesn't work in IE and in Mozilla and Netscape it
opens too early, almost instantly when the user
arrives at the site.
Can anyone help?
The script I've used is below.
Thanks, Canaani (Webslave)

<script>
//specify page to pop-under
var
popunder="http://www.dolphinswims.co.uk/thanks.htm"
//Pop-under only once per browser session? (0=no,
1=yes)
//Specifying 0 will cause popunder to load every time
page is loaded
var once_per_session=1

///No editing beyond here required/////

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)
win2.blur()
window.focus()
}

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

</script>