ti faccio un esempio
codice:
<form>
<input type="radio" name="NomeRadio" value="select" onclick="this.form.NomeSelect.disabled=!this.checked;this.form.NomeText.disabled=this.checked;" checked>
<select name="NomeSelect">
<option value="1">uno</option>
<option value="2">due</option>
<option value="3">tre</option>
</select>
<input type="radio" name="NomeRadio" value="text" onclick="this.form.NomeSelect.disabled=this.checked;this.form.NomeText.disabled=!this.checked;">
<input type="text" name="NomeText" disabled>
</form>