Hola Vicente.
Prueba asi:
Código PHP:
<html>
<head>
<script language="JavaScript">
function borra(){
var aItems = document.getElementsByName("rad");
for (var i = 0; i < aItems.length; i++)
aItems[i].checked = false;
}
</script>
</head>
<body>
<form name="frm">
<table border="0">
<tr>
<td>
<input type="radio" name="rad" value="1">Valor uno
</td>
<td>
<input type="radio" name="rad" value="2">Valor dos
</td>
</tr>
<tr>
<td colspan="2" align="center">
<input type="button" name="res" value="Deseleccionar" onclick="borra()">
</td>
</tr>
</table>
</form>
</body>
</html>
Saludos.