What do you have in the zip files? Is it a backup of the database, files, or both?
All you need is a backup of the database, it’s possibly called something.sql
Please read over these instructions completely before doing anything. If you’re unsure where your database backup is, what PHPMyAdmin is, how to use FTP, or if anything else here confuses you, I suggest either paying someone to restore your backup for you, or else contacting your hosting company. Generally hosting companies provide a backup of your files and/or database.
1) Before you do ANYTHING, back up what you have currently, including the database.
2) Delete every file from your WordPress installation, but leave wp-config.php and wp-content/
3) Using PHPMyAdmin or whatever MySQL management tool your hosting company gives you, drop the tables in your database (do not drop the database).
4) Use your FTP client to upload the files for WordPress 2.5.1 (or whatever you had prior to the upgrade). Do not upload wp-config.php or wp-content.
5) Import your .sql file from prior to the 2.6 upgrade using PHPMyAdmin (or whatever database management tool your hosting company provides).
6) Stop using one click install. It takes only a few minutes to install WordPress, even for a non-technical user.
7) Stop using automatic upgrade. It takes only a few minutes time to upgrade WordPress, even for a non-technical user.