Independent testing on most of the major virus scanners consistently shows that none provide 100% virus and malware detection. Anti-virus programs are not fool-proof and are continually identifying and patching new viruses. They’re always behind: first comes the virus and then come the patches for the new viruses. In the interim, you are vulnerable. Network firewalls prevent unknown programs and processes from accessing the system but are unable to identify or remove anything from the system. Even with a 100% effective anti-virus program and impenetrable firewall, hackers still have multiple pathways to access your machine through high and low tech strategies like social engineering. Always take precautions when using the Internet or installing programs/applications, and make sure of all the latest security updates for your OS and browser.