Prevent form redirect OR refresh on submit?

Here:

function submitClick(e)
{
     e.preventDefault();
     $("#messageSent").slideDown("slow");
     setTimeout('$("#messageSent").slideUp();
     $("#contactForm").slideUp("slow")', 2000);
}

$(document).ready(function() {
    $('#contactSend').click(submitClick);
});

Instead of using the onClick event, you'll use bind an 'click' event handler using jQuery to the submit button (or whatever button), which will take submitClick as a callback. We pass the event to the callback to call preventDefault, which is what will prevent the click from submitting the form.

It looks like you're missing a return false.

In the opening tag of your form, set an action attribute like so:

<form id="contactForm" action="#">

Just handle the form submission on the submit event, and return false:

$('#contactForm').submit(function () {
 sendContactForm();
 return false;
});

You don't need any more the onclick event on the submit button:

<input class="submit" type="submit" value="Send" />