function liste(listName, listDest) {
	i = document.listDest.listName.selectedIndex;
	if (i == 0) return;
	url = document.listDest.listName.options[i].value;
	parent.location.href = url;
}

Calling :
$cbb = '<form name="Choix2"><fieldset>';
$cbb .= '<select name="Liste2" onChange="liste("Liste2", "Choix2")">'."\n";
$cbb .= '<option>S&eacute;l&eacute;ctionner</option>';
while($res = mysql_fetch_assoc($exe)) {
  if($res['id_user'] == $id_user) { $sel = 'selected="selected"';	}
  else { $sel = ''; }
  $cbb .= '<option value="'.$redir.'&id_user='.$res['id_user'].'" '.$sel.'>'.$res['us_nom'].' '.$res['us_prenom'].'</option>';
    $i++;
  }
  $cbb .= '</select>'."\n";
  $cbb .= '</fieldset></form>';