Hola:
Te pongo un ejemplo javascript
:
<html>
<head>
<script>
function validar(f, check) {
var valida = false;
var arrayCheck = f[check];
for (i = 0, total = arrayCheck.length; i < total; i ++) valida = (valida || arrayCheck[i].checked);
if (!valida) alert('no hay marcas');
return valida;
}
</script>
</head>
<body>
<form method=get action="prueba.html" target="_blank" onsubmit="return validar(this, 'n[]')" >
<input type="checkbox" name="n[]" value="1" />
<input type="checkbox" name="n[]" value="2" />
<input type="checkbox" name="n[]" value="3" />
<input type="checkbox" name="n[]" value="4" />
<input type="checkbox" name="n[]" value="5" />
<input type="submit" />
</form>
</body>
</html>
Saludos
