This problem occurs because your user account doesn't have write permission to the data files. You either need to give your user account the proper file permissions or re-install FlashFXP and select a location for the data files where your user account has the proper file permissions.
This problem occurs more often than not when you've used a previous FlashFXP beta installer that let you install FlashFXP with default settings that were not vista friendly.
When installing FlashFXP you need to make sure that your data folder has read and write access. The best way to know this is to set your data folder to the recommended windows location, the user application data folder.
The following thread has an demo video showing how to install FlashFXP and make sure it's storing the data files in your application data folder.
http://www.flashfxp.com/forum/showthread.php?t=12958
You can check the location of your FlashFXP data folder from Within FlashFXP, from the menu > Help > About > Details tab
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