If you've had a few hackers as you mentioned, then yes they definitely could have caused the problem, and they still may have some exploit scripts on the server.
At this point, the safest thing to do would be to, at the minimum do what Ruben suggested, but do it in a clean directory. If you do an install/upgrade in your current directory you may end up with the same issues if you have an hacker uploaded files in there.
If you need assistance with this, you could order an Upgrade Install, and we could handle everything. Install in a new directory, upgrade your forums, clear out the old directory and then move it over to the current.
