There was a time not so long ago when you could barely open a Web site without being buried with pop-up ads -- unwanted windows advertising everything and anything up to and including the kitchen sink.

They existed for one very good reason: Annoying as they were, they worked.

Then, the Web browser makers got wise to the ways of pop-up ads, and enabled people to block them. Even Microsoft Corp., which was way behind the curve on this one, finally added the ability to stop most common pop-up ads to Internet Explorer with its Service Pack 2 update to Windows XP.

Unfortunately, that wasn't the end of pop-up ads.

Now advertisers have begun figuring out new ways to evade pop-up restrictions, and companies with goods and services to sell have been more than happy to place ads via these new channels, resulting in unpleasant surprises for users of such pop-up-blocking browsers as IE, Firefox, Safari and Opera.

This new crop of intrusive ads comes in two forms. One, pop-unders, appear behind the page you're viewing instead of in front of it. While that saves you from having your Web experience disrupted by a series of Viagra ads exploding in front of your nose, it still leaves you with an ad in front of your face when you close a browser window.

These pop-unders make it through pop-blockers by slightly changing how they create new windows, although they rely on the same basic JavaScript Web code used by traditional pop-ups.

(JavaScript also routinely used to provide legitimate Web site functions; blocking it entirely would render some sites unusable.)

A second kind is not launched with JavaScript code, but from components of a page created using Macromedia Flash. This technology allows Web designers to include things like interactive menus or games, but, like JavaScript, can also be deployed to create and present ads.

Some Flash ads have taken the form of "floaters" that appear to flip out in front of a page's contents when you arrive for up to 30 seconds. Since they don't appear in a separate window, they hide everything underneath them -- although many floater ads include a "close" button to dispel them.

More recently, however, advertisers have also begun to use Flash content to launch pop-up windows that look and act as they did in the bad old days, but which are immune to conventional pop-up blocking restrictions.

These souped-up pop-ups from such ad agencies as Tribal Fusion Inc. and Eyeblaster Inc. may present just text and graphic ads, but some may also include small Flash animations to attempt to draw your attention.

The developers of browsers have been reacting to these problems and revising their products to stop a revival of pop-ups. Internet Explorer (as updated by SP2) and the version of Safari in Apple's Mac OS X Tiger resisted these newer pop-ups in our tests, while Firefox's developers are working on an update to add this defense.

Why do companies go to the extra trouble of putting a pop-up or pop-under ad in front of users who, by choosing a pop-up-blocking browser, obviously don't want them?

Because people respond to them anyway, say companies that buy these types of ads.

"They work," said Susan Wade, spokesperson for Herndon-based Network Solutions Inc. "While they're only a small part of our media mix, they do help."

Baltimore-based Advertising.com's senior vice president, Jeremy Helfand, echoed Wade's assessment.

"All other things being equal, they're two or more times effective than static banner ads," Helfand said. But, he added, interactive ads that don't launch in separate windows can work just as well. "You can design them so that customers can interact with them to get a coupon or to explore a site without having to hit them with a big pop-up."

If online ads get too annoying, they will backfire on advertisers. But it's not clear yet if the latest proliferation of pop-ups has reached that point.