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9. Consumer-grade antivirus
Some business-class antivirus tools actually work — Symantec Endpoint Protection is one. It seems no matter which antivirus software you have, something is going to get through. Most consumer-grade antivirus does more harm than good. One of the exceptions I've found to that rule is Microsoft Security Essentials. Apart from that, your best bet is to either use OS X or Linux or unplug the PC from the network.