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<script language="javascript">
function to(form)
{
var myindex=form.menu.selectedIndex
if (form.menu.options[myindex].value != "0")                                  
  {
     window.open(form.menu.options[myindex].value,
     target="_blank");
  }
}
//-->
    </script>

<form name="nav">
  <select name="menu" onchange="to(this.form)" allowtransparency="true">
    <option selected>---</option>
    <option value="http://www.google.de">test1</option>
    <option value="http://www.google.de">test2</option>
  </select>
</form>