At my client site, I am not allowed to install software on my PC. While spyware or adware may install itself when I visit an evil web site, I cannot add software to thwart it. Consequently, my PC has become a nightmare of intrusive advertising.
For example, every time I log on, a message box tells me that spyware has been detected and asks if I would like to remove it. If I say "yes," another box tells me to pay $19.95 to activate this "helpful" software. Do people really trust this stuff to work? This program installed itself (along with an adware program) when I visited a rogue web site, and now it's offering to protect me from its brethren? Even more annoyingly, this box pops up four times before it leaves me alone.
Another piece of adware generates a pop-up ad every third or fourth time that I click on a link (any link at any site). There are only about half a dozen different ads that pop up over and over. The pop-ups for dating web sites are bad enough, but the worst pop-up ad I have seen asks, in big letters, "Should marijuana be legalized?" To the left of these words is a huge leaf. Yeah, that's just what I want on my screen when the boss walks past!