I know
I got experience with this so I acted a bit rough and suspicious in the start with him. After a while I gave it a shot and he actually helped me out and taught me how to prevent and now It's fixed.
The hacker changed two templates; "FORUMHOME" & "FORUMDISPLAY"
The hacker also used one of the admin's account to do this, cuz u could see a change in the teamplate from that account. The guy that helped me told me that he is being infected or someone know the password to the admin account..
Well after changing FORUMHOME & FORUMDISPLAY back to normal, everything works fine.. The only problem is that every "Custom modification/configure" to the templates or added custom pages was deleted...