[Dean C]

It's not that simple descds. This rule redirects everything (which is a bad thing), but I won't get into a discussion on poor usage of mod_rewrite now:

RewriteRule ^(.*) index.php

This will send every single URL you type to index.php. Why it's doing that I have no idea. This should be a temporary fix for you though:

 Options All -Indexes
Options +FollowSymLinks
RewriteEngine On
RewriteBase /

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^f([0-9]+)-([A-Za-z0-9-]+).html$
RewriteCond %{REQUEST_URI} !^t([0-9]+)(((-p)([0-9]+))?)([A-Za-z0-9-]+).html$
RewriteRule ^(.*) index.php

RewriteRule ^f([0-9]+)-([A-Za-z0-9-]+).html$ forumdisplay.php?f=$1 [L]
RewriteRule ^t([0-9]+)(((-p)([0-9]+))?)([A-Za-z0-9-]+).html$ showthread.php?t=$1&page=$5&pp=10 [L]

Give it a try anyway