$ is not defined - asp.net MVC 4

You've placed your script in the view body and not inside the Scripts section which is where it belongs and after referencing jQuery:

@section Scripts {
    @Scripts.Render("~/Scripts/jquery-1.7.1.min.js")
    @Scripts.Render("~/Scripts/jquery-ui-1.8.20.min.js")
    @Scripts.Render("~/bundles/jqueryval")

    <script type="text/javascript">
        $(function () {
            $('#cb').click(function () {
                if (this.checked) {
                    $('div#div').slideDown();
                } else {
                    $('div#div').slideUp();
                }
            });
        });
    </script>
}

Also to avoid having referenced jQuery twice (as in your case) double check if you haven't included it already as a bundle in your _Layout.

And one last remark: since by default scripts are included at the end of the DOM, just before the closing body tag, you don't need to be wrapping your script in a $(document).ready simply because by the time it executes the DOM will already be loaded. Your code is even worse because you had it twice. Bear in mind that $(function() { ... }); is equivalent to $(document).ready(function() { ... }); and in your case you've nested 2 of those things when you actually don't need any of them.

So:

@section Scripts {
    @Scripts.Render("~/Scripts/jquery-1.7.1.min.js")
    @Scripts.Render("~/Scripts/jquery-ui-1.8.20.min.js")
    @Scripts.Render("~/bundles/jqueryval")

    <script type="text/javascript">
        $('#cb').click(function () {
            if (this.checked) {
                $('div#div').slideDown();
            } else {
                $('div#div').slideUp();
            }
        });
    </script>
}