<script language=javascript>
function link()
{
if (document.activeElement.value == 1)
{document.my_form.action='index.php';}
if (document.activeElement.value == 2)
{document.my_form.action='forum.php';}
if (document.activeElement.value == 3)
{document.my_form.action='mail.php';}}
</script>
<form name="my_form" action="index.php" method="post">
<select name="type" onchange="link()">
<option value="1">Сайт</option>
<option value="2">Форум</option>
<option value="3">Почта</option>
</select>
<input name="login" type="text" />
</form>