In this article, the author explains how antivirus programs work. He also explains how performances of various antivirus programs vary from one to another. He also takes a look at 13 antivirus programs and explains which of these will keep computers protected. These programs include: (1) Sophos Anti-Virus Version 3.86.2; (2) McAfee VirusScan 9.0; (3) eTrust Antivirus Version 7.1; (4) Kaspersky Anti-Virus Personal 5.0; (5) PC-cillin Internet Security 2005; (6) Panda Titanium Antivirus 2004; (7) F-Prot Antivirus for Windows Version 3.15b; (8) Norton AntiVirus 2005; (9) F-Secure Anti-Virus 2005; (10) BitDefender 8 Standard; (11) NOD32 Version 2; (12) Norman Virus Control Version 5; and (13) RAV AntiVirus Desktop Version 8.6.