Mark wants to know what anti virus software to use, and is wondering if Avast is any good. Leo says that Avast is "freemium," which means he can use it for free or pay for more functions. Leo doesn't recommend it. For free, he recommends Microsoft Security Essentials. If he doesn't mind laying out a few bucks, Eset's Nod32 is the best.
(Disclaimer: ESET is a sponsor)