<form name="test" action="index.php" method="POST">
<table>
<tr>
    <td colspan="2"><input type="radio" name="radioBtn" value="1" 
        onClick="document.test.foo.style.disabled = true; 
                 document.test.id_item.style.disabled = false;" checked> Выбрать из списка *</td>
</tr>
<tr>
    <td colspan="2"><dd /><select name="id_item"><option value="3">Element  1 </option></select></td>
</tr>
<tr>
    <td colspan="2"><input type="radio" name="radioBtn" value="2"
         onClick="document.test.foo.style.disabled = false;
                  document.test.id_item.style.disabled = true;"> Новый *</td>
</tr>
<tr>
    <td colspan="2"><dd /><input type="text" name="foo" disabled></td>
</tr>
</table>
</form>