error_aviso="You must accept the terms and conditions";

function comprobaciones_contacto() {
	var forms = document.getElementsByTagName('form');
	var formulario = forms[0];
	
	if (!formulario.check_aviso.checked) {
		document.getElementById('check_aviso').className ="caja_form_error";
		alert(error_aviso);
		formulario.check_aviso.focus();
		return false;
	}
	
	formulario.submit();
}