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Has your browser's home page changed suddenly in recent months? Does your desktop sport a toolbar you don't remember asking for? Is your system tray crowded with mystery applications? You're not imagining things: Online advertising is more cunning, aggressive, and infuriating than ever. More than 25 percent of top Web destinations now use some kind of in-your-face marketing tactics, according to the Internet research firm Cyveillance.

Worse, corporate America is financing some of these intrusive ad campaigns. A PC World investigation shows that in today's complex Web economy, even reputable companies such as Citibank, Ford, and Sears can wind up paying commissions to aggressive Web marketers--often without realizing it. And since the marketers have little or no government regulation to hamper them--and enjoy the powerful incentive of lucrative ad budgets to underwrite their costs--the new and aggravating practices aren't likely to go away anytime soon.

Perhaps you've experienced some of these tactics, such as InternetFuel's method of pelting you with pop-up advertisements as you leave a site, or Search-Explorer.com's mouse-over downloads that can cause software to be downloaded to your PC's hard drive after you merely roll your pointer across an advertisement. Programs from Brilliant Digital Entertainment, Cydoor Technologies, and Gator may ride along when you install downloaded file-sharing software or Internet utilities. Then there are those InternetAlert pop-up advertisements: They look like Windows' system warnings.

There's nothing illegal about these actions, and they don't run afoul of any government regulations. Rudy Grahn, an analyst for Jupiter Research, says advertising regulations--which historically have been aimed at broadcast and print media--simply haven't yet caught up with the latest online strategies.

Yet the targets of aggressive marketing, including consumers and businesses, must contend with the adverse impact on PC and network performance. And irritated Web surfers have made their feelings abundantly clear on gripe sites. "What I don't understand is why this...isn't illegal," wrote one typical poster at Spywareinfo.com, which tracks online privacy issues. "Seems like a clear form of cyberterrorism to me."

Fueling this ad explosion is an estimated $9.6 billion that the GartnerG2 research firm says will be spent on Internet advertising in 2002. Web sites can now earn bounties for snagging your attention, your browser, your hard drive, or your name. For example, sites can earn up to 5 cents for each visitor who installs marketing software from the Webmaster Revenue Network, according to the company's Web site. They can collect as much as 20 cents for learning your zip code and e-mail address, say people who are familiar with the industry. And Web sites that ran those ubiquitous advertisements for X-10 wireless cameras may have earned up to $45 for each $90 sale they generated, according to Pesach Lattin, who is the author of the online marketing e-newsletter Adbumb. Adds Lattin: "The reason there's an upsurge in advertising sleazeware is because it works."

You Don't Even Need to Click

While most marketing software requires the user to click his or her assent before it installs, some newer technologies bypass this step. Earlier this year, for example, sites ran an ad that automatically downloaded a toolbar from Search-Explorer.com if the user moved their mouse over the ad while their browser was set to a low level of security. AdPowerZone, which created the toolbar, says it has the ability to track "every Web site the user visits, allowing our advertisers to send special offers to our users in real time while they are online." About 1.3 million people downloaded the software over a four-week period, the firm says.

AdPowerZone's president, Yves Lavoie, says that by mid-July only one Web site was still offering the mouse-over download. He notes that automatic installs occurred only if users had set their Web browsers to allow them.

Similar browser settings may enable some of Bonzi Software's pop-up ads to create a directory on your hard drive and download the company's marketing mascot, an animated purple gorilla that pitches to you whether you are online or off. "All we are trying to do is grab your attention the same way the employee outside Wal-Mart does by telling you what's on sale as you walk in," says Bonzi's John Epstein.

Britain-based C2 Media's MP3 Search application, which is distributed by sites such as MP3Search.com, promises to help you locate digital music. When we installed the software in April, however, it also switched our browser home page and default search engine to the Lop.com Web site. A Lop.com toolbar--with ads for Citibank, the Columbia House Record Club, Ford, and Sears--appeared, as did 89 new bookmarks, many of which pointed to Lop.com. And landing on Lop.com triggered a flock of pop-up and pop-under ads.