The system corruption must be documented by a verifiable computer/IT expert with a Microsoft license, detailing the precise damage that was done and by what method or agent. Still, he may sue you, and force you to incur legal fees, but unless he has documented proof, it would be a frivolous lawsuit.
If he has a good investigation done of his computer problems, they will most probably find that it was the (latest).worm or its ilk. Suggest to him, as a gesture of generosity, that he research sites such as
http://www.securityfocus.com/virus
or
http://securityresponse.symantec.com...ml#threat_list
before expounding on his computer knowledge and making accusations.
The very best firewall is Tiny Personal Firewall v5 but is it not for a novice (or an expert, for that matter). McAfee and Norton are extremely effective and user friendly, but you need the full complement of tools offered by these venders to have a fully protected computer.