There's a useful list of dodgy anti-spyware software
here.
Tactics seem to range from 'false positives' to encourage you to buy, to browser hijacking, aggressive advertising and even installation of trojans, spyware and adware. Domain name games seem common, too - for example, if you go for the well-regarded Spybot Search & Destroy, the the .com domain contains something very different to the real
http://www.safer-networking.org/ domain. And there's even
Google advertising to point you to the bad guys...
