Jquery doesn't work but javascript does…

Hi

I’m not sure what the problem is with my blog and jquery. I have the following code in my header.php that’s supposed to load jQuery:
<?php wp_enqueue_script("jquery"); /* Loads jQuery if it hasn't been loaded already */ ?>

When I view the page source I see that jQuery has been loaded:
<script type='text/javascript' src='http://domain-goes-here.com/wp-includes/js/jquery/jquery.js?ver=1.7.1'></script>

But I cannot perform any jQuery on my blog. I know it’s not a conflict because I created a blank html page and referenced the url in the page source and tested my jQuery in this html page and it didn’t work.

Then when I referenced a jquery file I have directly my jQuery code works fine.
<script type='text/javascript' src='js/jquery-1.5.1.min.js'></script>

This leads me to believe there is something wrong with WordPress’ jQuery file?

Any help would be greatly appreciated.