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Questo è il codice completo del sito da te postato...
Studiarlo un poco XD....

<!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" xml:lang="it-IT" lang="it-IT">
<head>
<meta http-equiv="Content-Type" content="text/plain; charset=utf-8" />
<title>JavaScripter</title>
<script type="text/javascript">
/* <![CDATA[ */
function checkBox(id, cbs) {
var cmd = document.getElementById(id) || document.all[id], n = cbs.length;
cmd.onclick = function() {
var status = this.checked;
for(var i = 0; i < n; i++) {
cbs[i].checked = status;
}
};
for(var i = 0; i < n; i++) {
cbs[i].onclick = function() {
var x = 0;
for(var j = 0; j < cbs.length; j++) {
if(cbs[j].checked) {
x += 1;
}
}
cmd.checked = (x == cbs.length) ? true : false;
};
}
}
window.onload = function() {
checkBox('c', document.forms.lol.a);
};
/* ]]> */
</script>
<style type="text/css">
/* <![CDATA[ */
/* ]]> */
</style>
</head>
<body>
<form name="lol">
<input type="checkbox" id="c" /><label for="c">Cliccami</label>

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

<input type="checkbox" name="a" />

Torna in JavaScripter</body>
</html>