I messed around with the forum stylesheet, and I discovered that the problem seems to be that IE requires the width of the <div> to be specified in order for the scroll bars to appear.

If you paste this code into a .html file, and then view the file with IE and FF, you should see the scroll bars in IE (where they weren't before).

Code:
<style type="text/css">
<!--
.ubbcode-block {
margin: 5px 2em;
background: #FFF;
border: 1px solid #AAAA11;
padding: 0px;
border-spacing: 0px;
}
.ubbcode-header {
display: block;
font-weight: bold;
padding: 4px;
color: #666666;
background: #EEEECC;
border-bottom: 1px solid #AAAA11;
font-size: 8pt;
}
.ubbcode-body {
margin: 4px;
//width: 100%;     //CHANGED HERE//
max-width: 100%; 
display: block;
max-height: 300px; 
overflow: auto; 
font-size: 10pt;
}
//-->
</style>

<div class="ubbcode-block"><div class="ubbcode-header">Code:</div><div class="ubbcode-body ubbcode-pre"><pre>

####
INSERT MANY LONG LINES HERE
####

</pre></div></div>


Notice the modification marked in the ubbcode-body definition. I've never used UBB forums before, so I don't know how hard it would be to modify the style sheet template. The // before the style should allow compliant browsers to continue working properly.

-genius_at_work