NO anti-virus can ever be relied upon to give any where near 100% protection. None of them do, free or paid. Most only detect about 90 to 95% of all KNOWN viruses. When a brand new virus is released, NO signature-based anti-virus will detect it for several hours up to several days. Some never will detect a particular virus. And over 1200 new variants of viruses and other malware are created by the "black-hats" each and every day. You should consider an anti-virus program to be your "last line of defense". Safe internet surfing and downloading practices (NO warez, NO P2P, NO Limewire, NO torrents, sorry) along with maintaining your Windows and all your installed software with all critical updates and security patches should be your frontline defense.