Typical Avast false positive. Google search and you'll see a lot of people complaining about this issue.
For what it is worth, I ran UF across Virustotal and only Avast picks up anything.
http://www.virustotal.com/file-scan...ceba510d7f6220d7e2dc9d236ced694539-1302552969
edit1: Looking at the forum.php source code, it's pretty standard stuff. Only one external script is called (google api script for something). Everything else is loaded from this site and appears to be standard VB scripts.
edit2: I would be more concerned if the forum had advertisements all over it, as that is a common way to get malware now (via outdated adobe flash and other security vulnerabilities on user's computers). But no ads here. TY fly for keeping this place ad free.