I really like Malwarebites MBAM and I have a paid subscription for $32.00CAD:
http://www.malwarebytes.org/mbam.phpI tried a-squared ages ago but it had too many false positives and I do not use Spybot S&D anymore as it only found tracking cookies and its scan takes unbearably too log to run.
I do use Microsoft's Windows Defender manually updated daily because of its monitoring of the HOSTS file and other key operating system files but I wish that its update function would work more reliably.
I also like WinPatrol and I purchased its PLUS update for $29.95US:
http://www.winpatrol.com I like the comfort of its mascot
Scotty's woof alerting me when things are changed on my systems.
[political rant]
Unfortunately, being on the Internet today has become such a target for the criminal element for their own nefarious purposes.
It is all in the name of gathering the almighty
$$$ as it is easy to steal as there is a very long time for them to be brought to justice or even to be caught. [/political rant]