Webmasters and advertisers in all industries are concerned about "scumware" aka "thiefware" that alters banner ads, link formatting, and or affiliate codes on users' computers.
From the player's perspective, "adware" aka "spyware" is the current hazard of the 'net. Spyware and thiefware are often linked, and may be gotten from the same despicable sources of infection. After all, the scumware has to have some information about the user in order to substitute "appropriately" targeted advertising.
The best article I have seen to date covering all these monstrosities of marketing can be found at:
It describes how to find and remove many different infections. Ad-Aware, by Lavasoft, remains the best infection protection for spyware that actively reports my computer use to some evil villains base.
I am also using Guard-IE, which pings me everytime a cookie is put on my drive or when a popup window has been thwarted. This lets me know which sites have no respect for me as a computer user. *ahem* It's better than the previous popup blocker I was using, because sometimes I do want to see a popup.
Popup, popunder, popafter, and "user response required marketing" (I have to find a close button) is extremely common in the online gambling industry as casino sites and banner farms augment their income by selling their "exit traffic" on the theory that since you came to a casino site in the first place, you are more likely to be a gambler and thus worthy of targeting.
Webmasters relying on these forms of advertising are also not going to recommend software that blocks poppies.
Using both products gives me a sense of power and surfing vindication. Every "ping" is an unhappy barbarian turned away from my gate.