direi cosi:
<html>
<head>
<title>Documento senza titolo</title>
<script>


function cambia() {
if (document.sax.html.checked = true){

alert (document.sax.html.value)
}
}

</script>
</head>

<body>
<form action="" method="get" name="sax">

<input type="checkbox" name="html" value="html" onClick="if(this.checked) cambia();"> html

</form>

</body>
</html>

come dici tu non capisco