[kh99]

Threads, posts, forums, users, etc are kept in the database, so you should be able to upload a fresh copy (of the exact same version) of vBulletin without losing anything, as long as you haven't modified the php files. There are some things that can be kept in the file system so you probably shouldn't delete any directories unless you're sure you're not losing anything.

ETA: it never hurts to have a backup of your database, so you might want to to that if you haven't already, then you could go about trying to restore things without worrying too much.