Upgraded it, now it nackered. can i deleete the whole forum and make a new one, but keep my boards, posts, members?
Sure, provided you point the new forum files to your old database as the database holds the members, forums, posts, polls, attachments, etc.
Remember, do not delete the attachments directory as well as avatars, you might want to retain Settings.php so the new files are automatically associated with the database.
See: Saving/Restoring, and Moving SMF From a Host to Another (http://docs.simplemachines.org/index.php)
-[n3rve]
And if you did not run upgrade.php when you upgraded before, make sure you run it after replacing all of your files.