Vedi se cosi va bene:
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">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<style type="text/css">
.stile{
	display: none;
	width: 600px;
	margin-right: auto;
	margin-left: auto;
	
}
</style>
<script language="javascript">
function mostra(obj,c){
var colore = new Array("#FF3300","#CCFF00","#990000")
var r = document.getElementsByTagName("input")
for(var i=0; i<r.length; i++){
if(r[i].id==obj){
document.getElementById('_'+i).style.display='block';
document.getElementById('_'+i).style.border=colore[c]+'solid 1px';
}else{document.getElementById('_'+i).style.display='none';}
}
}
</script>
<title>Documento senza titolo</title>
</head>
<body>
<form action="" method="get"> 
<input type="radio" id="ab" name="guida" value="abituale" onclick="mostra('ab',0);"/>Abituale

<input type="radio" id="dg" name="guida" value="seleziona" onclick="mostra('dg',1);"/>Seleziona

<input type="radio" id="ng" name="guida" value="nuova" onclick="mostra('ng',2);"/>Nuova

<div id="_0" class="stile">abituale</div>
<div id="_1" class="stile">seleziona</div>
<div id="_2" class="stile">nuova</div>
</form>
</body>
</html>