Viruses are malicious programs written and distributed to impair computer operation.  Spread by E-mail, Internet browsing, network connections and removable media, viruses can infect your computer without you knowing it.  Once a computer system is infected with a virus, that virus replicates itself to spread the infection.  Some viruses spread themselves through an entire computer hard drive, others propagate over network shares, and still others make their way into a computer’s boot sector and do irreversible damage to the hard drive.