Prueba esto:
Código HTML:
<form>
<input type="radio" name="rad" onclick = "document.getElementById('elBoton').disabled = false;" />
<input type="radio" name="rad" onclick = "document.getElementById('elBoton').disabled = false; />
<input type="button" id="elBoton" value="Algo" name="boton" disabled="disabled" />
</form>