Aside from the steps in the vBulletin documentation, the display message that is showing in place of the forum in vB5 is due to a template customization. Go to AdminCP -> Style Manager -> Edit Templates (select the one that is in use).
In the template editor, scroll and find Display Templates and double click it. Look for display_Forums. It will probably be highlighted in Red. Now, if you have customized this already, it will be red anyway. This is the file that contains the "Hacked by…" message. In my case, I just reverted to the default. There were no other changes I could find, but things may be different in your forum.