It could very well be a hardware related issue if it crashes in that fashion (files keep getting corrupt, over-heating, faulty power-supply unit, and so forth). Have you experienced any problems with other games, graphics intensive or not?

You could try disabling the opening movie via the FFXI Config Utility, to see if you get can get past that.

Moreover, if the game is installed via Steam, then only the PlayOnline Viewer/FINAL FANTASY XI via Steam should be used. In other words, if the game were to be installed via another medium, like CDs (yeah, anyone remember those? I have the 7 or so disc installation from 2004 I think it was, before the Treasures of Aht Urhgan even!), or a DvD, the easy way to move onward would be to remove that install, then re-install via Steam.

Installing the PlayOnline Viewer separately will indeed not work.