Prova questo.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<style type="text/css">
<!--
.href = {position:absolute;top:-1000;left:-1000}
-->
</style>
<script language="JavaScript" type="text/javascript">
<!--
function popup(str,nome,w,h,f) 
{ 
t = (screen.height-h)/2 
l = (screen.width-w)/2 
params = 'width=' + w + ',height=' + h + ',top=' + t + ',left=' + (l + f);
//alert(params);
msg = window.open(str,nome, params); 
} 

//-->
</script>


</head>
<body>
<input type="button" value="Cliccami" onclick="popup(document.getElementById('href1').href,'pippo',100,100,1)">

</body>
</html>
Comunque stai attento a mettere le parentesi prima e dopo l + f.
params = 'width=' + w + ',height=' + h + ',top=' + t + ',left=' + (l + f);