How to get an alert box after submiting a form

[cocotu]

At our website: chatwincompany.com
we want to have an alert window e.g "Thanks for Subscribing"
instead of having a new window coming from the 'insert.php' script. I tried may ways and I know this should be simple, but I'm a newbie at JavaScript. Below is the current code:

<form action="insert.php" enctype="multipart/form-data" id="form" method="post">
<div class="form"><img src="images/form_title_test.gif" alt="" /><br />
<div class="div_input"><input name="email" type="text" class="input_1" value="Enter email" /></div>
<a href="#" onclick="document.getElementById('form').submit()"><img src="images/link_subscribe_test.gif" alt="" /></a>
</div></form>

Thanks in advance....