Ho semplificato il tutto vedi se ti piace:
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="it">
<head>

  <title></title>
  <meta http-equiv="Content-type" content="text/html; charset=iso-8859-1" />

<script>
<!--
onload=function(){
if(!document.getElementsByName("agreecheck")[0].checked)document.getElementById("invia").setAttribute("disabled","disabled");}
-->
</script>

</head>
<body >



<div align="center">

<form name="agreeform" action="">
<fieldset><legend>Prova script</legend>
<textarea class="codecontainer" rows="8" name="S1" cols="45" wrap="virtual">Accetta le condizioni</textarea>

<input name="agreecheck" type="radio" onClick="if(this.checked)document.getElementById('invia').removeAttribute('disabled');" value='accetto'>Accetto

<input name="agreecheck" type="radio" onClick="if(this.checked)document.getElementById('invia').setAttribute('disabled','disabled');" value='nonaccetto'>Non Accetto

<input type="Submit" name="invia" id="invia" value="Submit!" disabled>
</fieldset>
</form>
</div>