soad, как вариант...
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>12345</title>
<style type="text/css">
</style>
<script>
function act() {
var o=document.getElementById('frm')
o=o.getElementsByTagName('input')
var val=document.getElementById('a').checked
var i
for (i=1; i<o.length; i++) {
o[i].checked=val
}
}
</script>
</head>
<body>
<form id="frm">
<input type="checkbox" onclick="act();" id="a" />
<br />
<input type="checkbox" id="b" />
<input type="checkbox" id="c" />
</form>
</body>
</html>