you can do it using javascript...
<script language='JavaScript'>
function OpenPopup(){
myWin=window.open('popup.aspx','Title','toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=yes,resizable=yes,copyhistory=no,width=480,height=500');
}
</script>
You can also generate a javascript function from code-behind using Page.RegisterClientScriptBlock()