The best free anti-virus software is either Comodo Anti-Virus or Advanced SystemCare 4.0. I've used both anti-virus/spyware removers to keep my system safe from viruses and on the odd occasion remove spyware and viruses that have crept past my firewall. You can actually use both of these systems on your computer at the same time, just so long as you're not already running Windows Firewall. which might cause a conflict.