If you accidentally install Pages and Structure together, do the following:
- Backup your database so you can restore the Structure tables later
- Make sure both Pages and Structure are uninstalled
- In your database, remove all tables prefixed with "structure_" and the Structure and Pages rows from exp_modules
- Delete the contents of exp_sites » site_pages
- Reinstall Structure
- Import your data from all "structure_" tables and exp_sites » site_pages
