You cannot save/load any file to the local disk of any user... if you could you would be a virus threat to the world. It is up to the user to download and install any such files you might offer for them to use on their local drive.
I have similar problem. We are producing a 70 meg browser based game and would like the user to be able to cache the game to avoid reloading the game the next time. Can this be done if the user authorise the save? If, so how? We don't care where the game is cached .
Can we prompt the user to download the file by providing a link from within the SWF application?
You might want to look into the Adobe Integrated runtime (AIR), with this runtime it is possible to save files to the usersharddisk