Try placeing this code in your header or where ever your menu is called from. You should have something like this right now?
<ul class="menu">
<?php wp_list_pages('depth=1&sort_column=menu_order&title_li='); ?>
</ul>
try making it this:
<div class="menu">
<ul>
<?php wp_list_pages('depth=1&sort_column=menu_order&title_li='); ?>
</ul><br />
<?php
if($post->post_parent)
$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0"); else
$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
if ($children) { ?>
<ul>
<?php echo $children; ?>
<?php } ?>
</ul>
</div>
By replacing the code you mentioned it changes the horizontal menu into a vertical menu,see http://blog.narrowminds.nl/ to view what I mean.
I have already tried drop down menu plugins but I could not find a plugin that is really easy editable (css) to make the lay-out the way I want it.