I don't have a problem with popup ads, really I don't. I will not think less of a company using them as a marketing technique. Of course, I use Google's popup blocker, so I don't see them any more anyway.
But what really chaps my ass is companies using plain old lies and deceit to sell their wares to an unsuspecting user.
For a prime example of this shady sales technique, I present the following popup ad (pop-under ad, technically) from a company called inKline Global:

inKline has gone through an awful lot of trouble to make this look like a page from Windows Update, even so far as creating a completely fictitous KB article number. (you can view the original page on inKline's site by clicking HERE, or view it through Google's cache) The problem is, clicking Continue doesn't download a necessary patch from Microsoft, it instead takes you to yet another deceptive page:

It looks like it's McAfee's Security Center site, but it's not. It's inKline still. Remember our talk about phishing a couple weeks back? Not everything is what it appears to be on the Internet. This page, when you scroll down, will eventually try to sell you their “Security Booster“ software product.
Here's a couple more examples of some recent popup ads people have been seeing lately, designed specifically to trick a user into thinking it's an update from Microsoft:

Remember kids, the Internet is full of people trying to seperate you from your money. And they will go to extraordinary lengths to do it.
(thanks Arturo Esquivel (via ntBugTraq) for providing the original images for some of the above examples)