так?
<html>
<head>
<script type="text/javascript">
function tyk()
{
var kn=document.getElementById('kn');
var kn2=document.getElementById('kn2');
var knopka1=document.getElementById('radio1');
var knopka2=document.getElementById('radio2');
(knopka1.checked)?kn.disabled=false:kn.disabled=true;
(knopka2.checked)?kn2.disabled=false:kn2.disabled=true;
}
</script>
</head>
<body>
<form id="form1" method=post>
<input type="radio" id="radio1" value="radio" name="radio" onchange="tyk()">radio
<input type="radio" id="radio2" value="radio" name="radio" onchange="tyk()">radio2</br>
<input type="button" value="knopka" id="kn" disabled>
<input type="button" value="knopka2" id="kn2" disabled>
</form>
</body>
</html>