No single antivirus software can defend against all viruses and trojans, and those that have been specifically designed to evade detection are even less likely to be caught by antivirus programs. Therefore, installing an antivirus isn't entirely foolproof. Moreover, many friends prefer to go without any protection (often referred to as "naked surfing"). Although HIPS (Host-based Intrusion Prevention System) is powerful, it is not without vulnerabilities. I recommend that those who like to surf naked use the portable version of antivirus software.