_install.php says:
You've entered the database name of the current, new installation. This install routine does not support install into the same database.
Please go back and either choose a different database name for your previous installation (you also have to rename your existing database) or choose a different database name for your new installation in the file lib/config.php.
Please refere to the INSTALL file in the root of your w3pw installation for preliminary steps required prior to coming this install script.
In the INSTALL file _install.php is not mentioned.