Are you trying to do a new install over an existing database? If so, you're generally supposed to do an upgrade unless you're migrating from UBB.Classic 6 or UBB.threads 6, in which case you have to be using either a different database or different table prefix (as compared to your old database) if your host limits your amount of databases.
For migrations you'd run the v7 installer (/install/install.php) in a different database or using a separate prefix as far as the createtable.php step brings you, then you'll have to run the import scripts (do not make an admin user or populate the tables).
For an upgrade within v7 you'd run the upgrade utility (/install/upgrade.php).
Could you outline what you're attempting to do, any steps you've taken, and what versions are involved?
