Conditional statements for advertising

[gfxhelp.com]

I currently have a system where an ad shows based on if it's a certain category. I have this setup and working, but if the visitor is not in a category such as home, search, memberslist etc etc, no ad is shown. I want to make it so that if the visitor isn't in one of the categories it shows a general "everything else" ad.

For example...I currently have

<if condition="in_array($forumid, array(1,2,3,4))">
<!-- Category 1 Ad Code -->
</if>
<if condition="in_array($forumid, array(5,6,7,8))">
<!-- Category 2 Ad Code -->
</if>

I have this setup for all my forums, but I need one more statement for everything else, for example:

<if condition="my forum">
<if condition="in_array($forumid, array(1,2,3,4))">
<!-- Category 1 Ad Code -->
</if>
<if condition="in_array($forumid, array(5,6,7,8))">
<!-- Category 2 Ad Code -->
</if>
</else>
<!-- General pages Ad Code -->
</if>

Any Ideas how to do this or something similar?
Thanks