The LATimes reports that Fortune 500 companies are among the unwitting backers of software that sneaks into computers and watches your online moves and/or pops open banners straight on your desktop, a.k.a spyware. Because ad-buying on the internet is often several steps removed, clients dole out a big lump of money to an agency who then passes this on to smaller agencies and buyers, who in turn pass it on to affiliates. The affiliates usually work on commision in a way, getting paid by clicks that they generate, so they'll try and generate clicks and viewings any way that they can - including by popping the ads into spyware. This is why Mercedes ads are mysteriously apperaing on your desktop when you never asked for it.
"The whole system seems like it's been designed to reduce accountability," said Ben Edelman, a Harvard graduate student who has testified before Congress on spyware practices. "It's a nightmare of backroom deals."