Hi,
Go with this code. I have created one function to check true or false to disable the textbox.

<script language="javascript">
function getValue(id) {
$strBoolen = (id==1) ? false : true;
document.getElementById("txtDate").disabled = $strBoolen;
}
</script>
<form>
Active <input type="radio" name="rdStatus" id="rdStatus" value="A" onclick="getValue(1);" />
InActive <input type="radio" name="rdStatus" id="rdStatus" value="InA" onclick="getValue(2);" /> <br /><br />
Date <input type="text" name="txtDate" id="txtDate" />
</form>

If you have any doubts let me known.