If you are on this page:

http://www.xmarksthescot.com/forum/index.php?

(the Forums index page) and from the Navigation box choose Forums which has this url:

http://www.xmarksthescot.com/forum/$vboptions[bburl]/index.php?$session[sessionurl]

You will get a 404 error page.

Forums link should not be active on the Forums page right? A small snippet of php can fix that.