Doomtrigger,

Bungiefan is correct. FINAL FANTASY XI and its four expansions (Rise of the Zilart, Chains of Promathia, Treasures of Aht Urhgan, and Wings of the Goddess) are not available for download (outside of where offered as a digital download through certain retailers). If you need to reinstall FINAL FANTASY XI, you will need to obtain a copy of the installation media. Information on the various collections and links to retailers can be found on the official PlayOnline website.

The official PlayOnline website can be found at this link:
Official PlayOnline website