Roogle is right, they store all your character information on their servers. If you have deleted you Content ID then you can just re-activate it and get your character back... I think that's the same with the Playonline ID except you only have three months to retrieve it again.

If you uninstalled everything on FFXI and did not cancle any subscription in the Playonline Viewer, then you should be fine. Jsut re-install it again and you can still sign on.