Looking at the source of your forum it looks like you have some bad HTML displayed (possibly in your HTML includes, CP -> Display -> General -> HTML Includes), as an example:
<table class="center fw" style="padding:0;margin-top:-5px;">
<tr><td class="body_col fw"><table width="100%" class="t_outer" cellpadding="0" cellspacing="0">
<tr>
<td>
<table width="100%" class="t_inner" cellpadding="0" cellspacing="1">
<tr>
<td class="tdheader">
The Keyboard Corner
</td>
</tr>
<tr>
<td class="alt-1">
<tr><td align="center" valign="center" ><IMG SRC="http://www.ex5tech.com/ads/KC_banner_02_new_ani.gif" BORDER="0" ><br />
</td></tr></td>
</tr>
</table>
</td>
</tr>
</table>
Which looks like the category description, where your banner displays:
http://www.ex5tech.com/ads/KC_banner_02_new_ani.gifYou should try disabling any HTML includes or HTML in descriptions, and see if the problem goes away; then if it is the culprit, rewrite your HTML to not have additional misused tags (such as closing cells that are already closed).
Note that out of the box you shouldn't be having these issues (or we'd have several sites complaining of the same problem), so its important to troubleshoot by removing bits of added code until you see what is causing the behavior that you're experiencing.
It is important to also add that any modifications to the out of the box coding is completely unsupported. If you have a developer making changes to php or template files and making changes, there isn't anything we can do to support any 3rd party changes; this includes any coding you add to the HTML includes (as we only support the coding as it comes shipped in the install archive).
