Make a CMS like main-menu and sub-menu

Hi community!

I’m building a CMS based on WP. One thing I can’t make happen is my menu… Probably it’s very simple, but I can’t figure it out.

Simply, I want a main-menu with categories and pages in one list. When a category is clicked, I want a new list to appear with the sub-menu in (sub-categories)

You can take a look at my demo; my demo

It’s almost there, but when the sub-menu-item “Office” is clicked, the wholæe sub-menu disappears!

As it is now, I’m using this code;

<div class="grid_10" id="topMenu">
<ul>
<?php wp_list_categories('depth=1&sort_column=menu_order&title_li='); ?>
</ul>
<div class="clear"></div>
</div><!--topMenu-->

<div class="grid_10" id="subMenu">
<ul>
<?php if (is_category()) {
$this_category = get_category($cat);
if (get_category_children($this_category->cat_ID) != "") {
echo "";
echo "";
wp_list_categories('orderby=id&show_count=0&title_li=
&use_desc_for_title=1&child_of='.$this_category->cat_ID);
echo "";}} ?>
</ul>
<div class="clear"></div>
</div><!--subMenu-->

I know the sub-menu disappears cause the code looks for the children of the sub-menu-item. But I can’t figure it out…

Any help is appriciated…

Arnii