nraw,
<html>
<head>
<title></title>
<script>
function a() {
var n = document.getElementById("tbnum").value * document.getElementById("tbnum2").value;
document.getElementById("result").innerHTML = n;
}
function check(obj) {
obj.value = obj.value.replace(/[^0-9]/g,'');
if (obj.value.length == 1){
if (obj.value == "0") obj.value = "";
}
if ((obj.value - 0) <= 0) obj.value = "";
a();
}
</script>
</head>
<body>
<input type="checkbox" value="asd" id="tb1">
<input type="text" id="tbnum" onkeyup="check(this)">
<input type="checkbox" value="asd" id="tb2">
<input type="text" id="tbnum2" onkeyup="check(this)">
<p>Результат: <span id="result">0<span><p>
</body>
</html>