<script>
function pasif()
{
	document.getElementById("mk1").disabled=true;
}
function aktif()
{
	document.getElementById("mk1").disabled=false;
}
</script>

<form action=""> 

<table border="1" width="100%"> 
    <tr> 
        <td colspan="2"> 
        <input type="radio" value="V1" checked name="R1" id="rb1" onclick="pasif();">Seçili  
        Durumdayken Metin Kutusu Aktif Olmayacak</td> 
        <td width="653"><input type="radio" name="R1" value="V2" id="rb2" onclick="aktif();">Seçili  
        Durumdayken Metin Kutusu Aktif Olacak</td> 
    </tr> 
    <tr> 
        <td>Metin Kutusu</td> 
        <td> 
        </td> 
        <td width="653"><input type="text" name="T1" size="20" disabled="disabled" id="mk1"></td> 
    </tr> 
    </table> 
</form>