[fel64]

This isn’t about the files. This is about the databases.

You need to use phpMyAdmin or another tool to get rid of the old tables. If you do not know how to access your databases in this way, contact your host.

The “already installed” message occurs after bbPress checks your databases for an entry in, I can’t remember, bb_forums or bb_topics. While there is data in those tables, you cannot install again. That’s why you need to get rid of them. You also need to get rid of the other bb_ tables because it will try to create new ones with the same name.