I went on a personal vendetta after my customers were annoygin me with this misguided belief that free actually works, so I set up a little demonstration.
AVG does not recognize Blaster worm... a now archaic virus that there is no reason not to recognize;
AVG, Avast, MAlware Bytes, and AD-Aware all do not effectively block or remove Security Tool, the current most popular scare ware.
Not saying MB-aw is bad... just saying, it alone cannot be depended on. AVG is itself useless. Avast is a step better but still not worth the ram it uses to run.