Convertir rotador de popus en popunders

[josemi230479]

Hola buenas tengo el siguiente codigo que es un rotador de popus y no se como puedo convertirlo para que se abran detras de la pagina principal es decir popunder.El codigo es el siguiente :

Código:

<SCRIPT language=JavaScript>
/*
 +---------------------------------------------------------+
 | PopUp PopUnder and Exit Console Rotator  YugDesign.com  |
 +---------------------------------------------------------+
 | This program may be used and hosted free of charge by   |
 |anyone for personal purpose as long as this copyright    |
 |notice remains intact.                                   |
 +---------------------------------------------------------+
*/

var switchto = 1; // set to 1 = PopUp, 2 = Exit Console
var under = 0; // set to 1 to make it pop Under
var resize = 1; // set to 1 to resize main window

var url=new Array(); 
url[0]= "http://www.google.es";
url[1]= "http://www.yahoo.es";
url[2]= "http://www.msn.es";

//no need to change anything below this line
 
var id = Math.floor(Math.random()*url.length);  

function window_onbeforeunload(){if(switchto == 2) { window.open(url[id]); }} function window_onload(){ for(i=0; i<document.links.length; i++) { document.links.item(i).attachEvent('onclick', OnClick); } for(i=0; i<document.forms.length; i++) { document.forms.item(i).attachEvent('onsubmit', OnClick); }} function OnReset(){switchto = 2;} function OnClick() {switchto = 2;window.setTimeout(OnReset, 4000);} window.attachEvent('onload', window_onload); 
if(switchto == 2) window.attachEvent('onbeforeunload', window_onbeforeunload); 
if(switchto == 1) window.open(url[id]);
if(under == 0) self.focus();
if(resize == 1) self.moveTo(0,0);self.resizeTo(screen.availWidth,screen.availHeight);

</SCRIPT>