Finns olika varianter på lösningar, här är en. 
[red]<script type="text/javascript">
function check(objField)
{
var elements = document.getElementsByTagName('input');
for(i = 0; i < elements.length; i++)
{
if((elements[i].name.indexOf(objField)!=-1))
{
elements[i].disabled = false;
}
else
{
if(elements[i].value == '')
{
elements[i].disabled = true;
}
else
{
elements[i].disabled = false;
}
}
}
}
</script>
...
<form>
<input type="radio" name="bar" onclick="check('one');"> One<br>
<input type="radio" name="bar" onclick="check('two');"> Two<br>
<input type="radio" name="bar" onclick="check('three');"> Three<br>
<input type="radio" name="bar" onclick="check('four');"> Four<br><br>
One: <input type="text" name="one" disabled="disabled"><br>
Two: <input type="text" name="two" disabled="disabled"><br>
Three: <input type="text" name="three" disabled="disabled"><br>
Four: <input type="text" name="four" disabled="disabled">
</form>[/red]