
You frequently turn-off virus signature updates - This is clearly an invitation to infections from unknown viruses. Unless you keep your AV software's virus signature database up-to-date, you cannot guarantee protection from new and latest threats emerging almost every day. There's no reason to pause virus signature updates as they're done at most twice or sometimes thrice a day.
You install a conflicting firewall with it - Some enthusiasts love to experiment with different combinations of software. One such deadly combination is the installation of a dedicated firewall with an antivirus solution. For an average user, resolving conflicts between the two is not a cake walk. This conflict often creates loopholes that can be easily exploited by lurking worms and malware on the internet.
You never do full system scan - Typical antivirus software never perform full system scan automatically. They do prompt you to do the same and relies on your permission for performing a thorough analysis of the system. If you've never initiated deep scanning of all your hard drives, you're probably taming a silent time bomb deep down in some directory.
You selectively disable shields temporarily - I've observed many users temporarily disabling one of the available shields in their antivirus software. They do it to quickly access any restricted resource. This can prove fatal in two ways. First, you cannot guarantee that the resource is going to be virus-free in your next attempt. And secondly, during this duration (disabled shield), there're very high chances of infection that may go undetected due to inactive shield.
You fail to follow instructions after the alarm - And last but not the least is axing the very same branch on which you're sitting. Sometimes users' acts dump as soon as a virus is detected and the AV software prompts with a warning. Generally, this is accompanied with options that can be followed to neutralize the virus. Some of us quietly ignore the instructions or do not follow them properly. This result in instant infection which is later blamed on the antivirus software.