Playonline update issues

I'm running Windows Vista 64 bit, and downloaded the vista compatible playonline viewer from the website. Despite that, whenever I try to update playonline after doing a clean uninstall, it tells me, "Cannot replace the file path.bak" and proceeds to shut down playonline after giving me an error when the update is complete, saying an error occurred writing to the harddrive. Any help would be appreciated, as right now I've paid for the game but can't seem to get it running on my own computer! I've done all I can think of to fix the issue, and can only think that when I was trying to get it running via steam after downloading the trial version at some point something messed with the registry. Unfortunately I'm not advanced enough to deal with any issues like that. Once again, any help at all would be greatly appreciated!

We are sorry to hear that you are having installation issues with FINAL FANTASY XI. Due to all the uninstall and reinstall attempts, we would like to suggest uninstalling all components of FINAL FANTASY XI, downloading and running a registry cleaner, and reinstalling FINAL FANTASY XI. If this does not fix the issue, please answer if you are currently trying to run the game on Steam or from another installation media.