If it still does this after checking for corrupt files, all you can really do is uninstall FFXI and PlayOnline Viewer, then reinstall them from scratch (make sure you write down your PlayOnline Username and password before doing this).
Otherwise, if this is your first time playing FFXI, it could be your computer isn’t powerful enough to run FFXI, although usually it would throw up an error message stating that if it was the case.