<script language="javascript">

function Scegli(){
var oSelect = document.getElementById("scegli");
var oText = document.getElementById("testo");

oText.value = oSelect.options[oSelect.selectedIndex].value;

}
</script>

.
.
.

<SELECT NAME="scegli"
onchange="Scegli()">
<OPTION VALUE="Books">Books
<OPTION VALUE="Clothing">Clothing
<OPTION VALUE="Housewares">Housewares
</SELECT>
<input type=text name="testo">