I recently had a rather rampant piece of Adware that was effecting Chrome, and causing several miscellaneous words to underline, and hyperlink their way to ad sites. I eventually found the location of the executable, and submitted it to Windows Defender (The Anti Virus I was using at the time).
What I got back shocked me. A result of "Not detected". This means that the executable had been previously submitted, and had been found by the Microsoft researchers to not be harmful.
Curious about the result, I decided to submit the same file to several online virus scanners - All of which detected the file as harmful by no less than 15 separate anti virus scanners, none of which were Windows Defender.
Curious, I decided to check some previous submissions of mine. The early ones (Back in the Windows 7 / Early Windows 8 period) had all subsequently been added within a few days of my submission. The later ones were either Not Detected or simply No Scan Result Available. Keep in mind that both of those submissions are picked up by the majority of other Anti-Virus's (AVG, F-Secure, Malwarebytes, McAfee, Etc), so I decided to switch.
Having 3 known virus files at my disposal, I decided to look around to various free alternatives. I used to use AVG when they were still located at http://free.grisoft.com/ before they went corporate, but they have since severely dwindled in quality. Norton was out of the question (Any tech-savvy person will know how useless and bloatware-esque it is), so I decided to try Malwarebytes. I downloaded their free version located here (16.5MB), and it quickly scanned my PC, effortlessly finding the 3 files I had, as well as some registry entries that the Adware had created. Happy, I acquired a premium version (Real Time Protection), and went merrily on my way.