AFAIK you can't install ASB on Vista or XP. In any case, Avast SafeZone Browser is not supported anymore also.
Would seem to be the reason why you have the update error you noted in your first post.
Vista is EOL (End Of Life) as is XP and not supported anymore. The only avast browser you can run on Vista is Avast SafeZone browser, and that is EOL too.
>Support for all operating systems does not last forever.<
When support ends, so does running a secure system, that ends when support ends too.