ok, basta questo
codice:
...
<input type="radio" name="nomecheck" value="si" onclick="this.form.nomeselect.disabled=false" />sì

<input type="radio" name="nomecheck" value="no" onclick="this.form.nomeselect.disabled=true" />no

<select name="nomeselect" disabled="disabled">
<option></option>
</select>
...
ciao