I have a large forum that has been hacked and infected by a trojan. The trojan would automatically download onto the surfer's computer and crashes it. I restored a backup made a couple of weeks ago but it seems that the trojan was already there. The earliers trojan-free backup I have got is 6 months ago. I wonder if there is anyway to get rid of this trojan without having to restore this rather old backup? Any opinions would be appreciated.
I assume your forum content is in a database? So try deleting the php or whatever files and then fresh installing them. It's unlikely that the trojan is in forum posts / user data.