There is a major problem in the world of spyware/adware scanners; they all try to look the "best" by scanning the "most", and in the course of events they'll set off alarms for all kinds of stuff. Most of them claim that cookies are bad (they are not), many of them even set off alarms when they find a banner in your browser cache. REAL spyware is nasty stuff, it isn't a banner and it isn't a cookie; it is stuff that captures keystrokes, it sends whatever you do to someone else, and it allows others to hijack your PC to send spam. Adware is as vicious but its main intent is to use your PC and your browser to force you to see their ad's. This could mean any time you enter a URL it takes you to their search engine, or any time you enter something in Google it pops up an ad from them instead.

Windows defender (to me) is the best because it just sticks to the nasty stuff, it doesn't try and alarm you when it finds a cookie

On my PC I scan once a week, sometimes twice. I am not afraid of spyware/adware because I use Firefox and Thunderbird, plus I keep an eye out for downloads I know may not be legit. Despite all the hyperbole from the "fruity" side of the PC world it really is easy to prevent this stuff. I know people that run 2 different virus scanners, 2 spyware scanners, firewall software AND still manage to open stuff they shouldn't (someone I know brings his PC to me at least once a month to have it cleaned, despite all the scanners he installs).

Common sense is the best scanner, that and always installing the most recent windows updates.

Windows Defender used to be a product from a different company, but I guess Microsoft felt so guilty for all their bugs that they purchased this firm and not offer the product for free.