Nicole I am not sure how you could say that the virus was from this forum/site when there is no such logged proof.
I mentioned it before that viruses can spread through any kind of medium as long as it is "connected" to the world. What if the virus was from another website or a program or some sort of media (like usb, disc etc)?
One can't tell however, what one can do is to check the logs on the computer and see the source of the virus. Firewall won't stop the virus coming into your computer however would stop the website from opening in the first place. Antispyware/Antivirus would stop the virus from triggering if it is running under real time.
Additionally, In general there are so many links on the forums that upon clicking them you might be diverted and get infected however that is not the responsibility of the forum but the site where you have visited.
I am sure if your father in law had a good antivirus and spyware it would have stopped the virus/spyware coming into his computer.
As I said it could be anything unless or otherwise a log shows that the virus was spread through the forum, one cannot explain where the virus came from.
If something's HARD to do, then its NOT worth doing!