prova questo codice che scrissi qualche tempo fa:

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

<script language="JavaScript">

function Attiva(n, nomecheckboxses, f) {
 var c=f.elements[nomecheckboxses];
 for (i=0; i<c.length; i++) c[i].checked=(i==n);
}

</script>


</head>

<body>
<form action="" method="post" name="mioform" id="mioform">
  <input type="checkbox" name="checkbox1" value="c11" onClick="Attiva(0, this.name, this.form)">


  <input type="checkbox" name="checkbox1" value="c12" onClick="Attiva(1, this.name, this.form)">

  <input type="checkbox" name="checkbox1" value="c13" onClick="Attiva(2, this.name, this.form)">
</form>
</body>
</html>
ciao