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Not sure if this has been fully answered or addressed, or if the user asking the question was kidding. However, language is one of my fortes, if I do say myself though. A silver bullet is a metaphor for any straightforward solution thought to be extremely effective against problems when ordinary weaponry and combat methods fail.
In this case, avast! being the silver bullet means that it is being marketed as a product that will do the trick and fix your computer, in simplest terms.
