WordPress wants to install itself, rather than run the blog

WordPress redirects you to that installation screen because the database it’s connecting to is working (meaning, the username and password are correct), but the data it’s expecting to be there isn’t. Therefore, it assumes it’s a new / empty database and prompts you to install WordPress.

I’ve seen this happen in two scenarios:

1. The database really is empty, and thus WP needs to install the standard tables and info
2. The table prefix in your wp-config.php file is incorrect for an existing database

Look at your wp-config.php file in the root directory of WordPress, and look for a line similar to this:

$table_prefix = 'wp_';

Then, open up the database (phpMyAdmin or some other interface to browse what the database structure actually is) and confirm that the table prefixes (the first few characters of the table names) actually match what’s set above.

Hopefully this gives you something to go on! Let us know what you find