Sorry for being so late at the party, but I see some room for improvement here. Not concerning "disable textbox", but to the radionbox selection and code simplification, making it a bit more future proof, for later changes.
First of all, you shouldn't use .each() and use the index to point out a specific radio button. If you work with a dynamic set of radio buttons or if you add or remove some radio buttons afterwards, then your code will react on the wrong button!
Next, but that wasn't probably the case when the OP was made, I prefer to use .on('click', function(){...}) instead of click...
http://api.jquery.com/on/
Lst but not least, also the code could be made more simple and future proof by selecting the radio button based on it's name (but that appeared already in a post).
So I ended up with the following code.
HTML (based on code of o.k.w)
<span id="radiobutt">
<input type="radio" name="rad1" value="1" />
<input type="radio" name="rad1" value="2" />
<input type="radio" name="rad1" value="3" />
</span>
<div>
<input type="text" id="textbox1" />
<input type="checkbox" id="checkbox1" />
</div>
JS-code
$("[name='rad1']").on('click', function() {
var disable = $(this).val() === "2";
$("#textbox1").prop("disabled", disable);
$("#checkbox1").prop("disabled", disable);
});