hola, aqui esta este ejemplito:
Código HTML:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Untitled Document</title>
</head>
<script language="javascript">
function validar() {
var f = document.form1
var sw = false;
for (i=0; i < f.checkbox.length; i++) {
if (f.checkbox[i].checked == true) {
sw = true;
}
}
if (sw == false) {
alert("No has seleccionado ningun checkbox!!!")
return false
}
return true
}
</script>
<body>
<form name="form1" method="post" action="" onSubmit="return validar()">
<input type="checkbox" name="checkbox" value="1">
<input type="checkbox" name="checkbox" value="2">
<input type="checkbox" name="checkbox" value="3">
<input type="checkbox" name="checkbox" value="4">
<input type="checkbox" name="checkbox" value="5">
<input type="checkbox" name="checkbox" value="6">
<input type="submit" name="Submit" value="Submit">
</form>
</body>
</html>