Could be one of 2 things. The first would be to just try and run FF as an "Administrator", Right Click > Run As Administrator

If that doesn't work your boot file is probably corrupted which requires a re-install. Before you uninstall make a copy of the game folder as your game data is probably fine. Uninstall the game, re=install the game and then copy the game folder back. That will save you needing to download the update again.