Código HTML:
<div id="Layer1">
<form method="post" name="frmX" id="frmX">
<table border="0" height="114" width="413">
<tbody><tr>
<td colspan="2"><div align="center">Seleccione lo que Desea </div></td>
</tr>
<tr>
<td width="146">
<label>
<input name="radiobutton" id="1" value="individual" checked="checked" onclick="document.frmX.ini_letra.disabled=true;document.frmX.fin_letra.disabled=true;document.frmX.cant_letra.disabled=false;" type="radio">
Individualmente
</label></td>
<td width="257">
<label>Cantidad de Letras
<input name="cant_letra" id="cant_letra" size="5" type="text">
</label></td>
</tr>
<tr>
<td>
<label>
<input name="radiobutton" id="2" value="intervalor" onclick="document.frmX.ini_letra.disabled=false;document.frmX.fin_letra.disabled=false;document.frmX.cant_letra.disabled=true;" type="radio">
Por Intervalos </label></td>
<td>Entre
<label>
<input name="ini_letra" id="ini_letra" size="10" type="text" disabled="disabled">
y
<input name="fin_letra" id="fin_letra" size="10" type="text" disabled="disabled">
</label>
</td>
</tr>
</tbody></table>
</form>
</div>