These programs are only effective, if you are totally uninterested in computer security or to clean heavily infected machines. On the other hand, scanners are the only means to know if something malicious is already on your system. I check my systems only 2-3 times a year, and without sounding arrogant, not one single malicious object was ever found in 5 years.
Besides my Kaspersky suite I scan on a regular bases with SuperAntipsyware. Although it really never finds a thing
