One problem you are going to run into is that only IE4 and above support diabling form fields, but with that said, here is how you do it in IE.
You are going to have a Javascript function that is activated on the radio button's, onMouseUp event. The function will look at the value of the radio field and then look at the disabled value of the text box. If the radio button is "Yes" for example, and the text box is disabled, you enable it, and vice versa. Here is the code:
<SCRIPT LANGUAGE="JavaScript">
<!--
var IE4 = (document.all) ? 1 : 0;
function enableText() {
if (!IE4) return;
var form = document.myForm;
var currentState = form.myText.disabled;
var changeState = form.myRadio.value;
if (changeState != currentState)
form.myText.disabled = changeState;
}
// -->
</SCRIPT>
<FORM NAME="myForm">
<INPUT TYPE="radio" name="myRadio" value=true CHECKED onMouseUp="enableText()">Disable
<INPUT TYPE="radio" name="myRadio" value=false onMouseUp="enableText()">Enable
<INPUT TYPE="text" NAME="myText" SIZE="30" DISABLED>
</FORM>
You can get a more thourough description by following this link:
http://www.webreference.com/js/tips/991030.html