phpbb is the best free forum software that is. The fact that this virus spread so fast is a testament to the massive use of it on the internet. In that news.com post it said that their are voer 6,000,000 phpbb's out there. It has its flaws of course and the fact that its code is freely available makes it a good candidate for something like this.
Of course once you move up in needs you have to go to VB