Código Javascript
:
Ver original<script>
function f_activa(opc){
for(var i=0;i<document.forms[0].getElementsByTagName("input").length;i++){
document.forms[0].getElementsByTagName("input")[i].disabled=true;
}
document.getElementsById("txt_"+opc).disabled=false;
}
</script>
<form>
<select id="slct_txt" onchange="f_activa(this.value);">
<option value="0">Elige txt</option>
<option value="1">txt 1</option>
<option value="2">txt 2</option>
<option value="3">txt 3</option>
</select>
<input type="text" id="txt_1">
<input type="text" id="txt_2">
<input type="text" id="txt_3">
</form>
Saludos.