<html>
<head>
<script>
function MySubmit(){
if (!document.getElementById("MyCheckBox").checked){
alert("Devi selezionare la checkbox!");
return false;
}
else
return true;
}
</script>
</head>
<body>
<form id="MyForm" onsubmit="return (MySubmit())">
<input type="checkbox" id="MyCheckBox">
<input type="Submit" value="Prova">
</form>
</body>
</html>