Problem u JS sa alertom i confirm u IE

Code:

<script language="JavaScript" type="text/javascript">
<!--
function  upit()
{
var sl = document.getElementById("id_s").value;
var r=confirm("Zelite  Brisati sliku? ");
if (r==true)
  {
      document.location.href="brisi_sl_gal.asp?bris='da'&id='" + sl + "'"  ;
  }
 else
 {
    document.location.href="brisi_sl_gal.asp?bris='ne'&id='" + sl + "'"  ;
  }
    }

//-->
</script>

....................................
  <body> 
     <input type="hidden"  id="id_s" value="<%=id_slike%>"></input>