I'm trying to utilize a simple thickbox as with here: http://twostepmove.com/modules/mod_form/tmpl/demo/main_form.php

however when putting the exact same code inside a joomla module the hyperlink does not respond whatsoever. the code is:

<a href="#TB_inline?height=155&width=400&inlineId=modalWindow" class="thickbox">Click here    
for an autocomplete inside a thickbox window.</a> (this should work even if it is beyond   
the fold)
<div id="modalWindow" style="display: none;">
            <p>
...
            </p>
    </div>

so that as I stated it does not work on all when put in the joomla module. must i install some kind of wordpress plugin or something like that like this? thanks.

you will find lots of jqeury plug ins for Joomla, or place jquery script within the index.php file by hand. Take care not to mix along with other javascript frameworks, you can aquire a conflict

Is you will find reason you won't want to make use of the built-in lightbox? You can include class="modal" to some link and also the built-in lightbox work with no extensions.