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The most used free AV on the web are Avira, Avast, Bitdefender and more in TOP 5 Free Antivirus in 2018. Note that the major downside of these freewares is the ads bundled with them though they are fine and enough for protecting a personal or home computer, find different ways(like premium protection suite) if your mind that. Aside from antivirus, I also suggest you do a thorough cleaning to your system. Install a cleanning app like CCleaner if you do not have one on your pc yet, use it to clear up the trash and uninstall unnecessary apps and tools in the system. Developing a habit of cleaning trash on a regular basis is quite important to keep the system light and fast. In addition to that, remove all the unwanted files in C drive and avoid setting it as the destination of various downloads. Hope this helps!