anche così
codice:
<script language="javascript">
function setHref(valore){
	prova.href =valore
	prova.innerText =valore
}
</script>
<select name="pippo" onchange="setHref(pippo.options[selectedIndex].id);">
	<option id=""></option>
	<option id="prima.asp">prima.asp</option>
	<option id="seconda.asp">seconda.asp</option>
</select>
ciao