John Borland reviews the situation:

Those persistent types of programs, frequently operating on computers without owners' knowledge, have spread quickly in the last year, evolving as rapidly as anti-spyware software has been able to find them. EarthLink executives estimate that 40 percent to 50 percent of the Internet service provider's subscribers have running on their machines some kind of advertising or more-malicious program, which often monitors their behavior and sends the data back to the software's parent company.

....Security companies say that the incidence of so-called spyware, adware, sneakware and other varieties of surreptitious software is climbing dramatically, adding that the most irritating of the bunch are becoming even more difficult to stop--or even identify.

These types of programs had been available for years but became more common as free file-swapping services such as Kazaa and Imesh began bundling these ad-supported programs with their software to help pay their bills. Today, many programs are automatically installed when a person views an unsolicited HTML (Hypertext Markup Language) e-mail or visits Web pages that activate a "drive-by download."

The most benign of these programs simply serve advertisements. Others can collect detailed information about a viewer's behavior and send it back to a parent company the person likely knows nothing about. Many change the settings of a browser or other software, sometimes in ways that only someone with sophisticated technical knowledge can reverse.

....Recent months have seen a spurt in so-called browser helper objects (BHO), which attach themselves limpetlike to Microsoft's Internet Explorer browser software and act as a toolbar or other browser plug-in. The worst of these can radically change browser settings, including home pages and bookmarks, and make it difficult or impossible for people to change these back without their knowing how to manipulate the Windows registry. Recent examples of these, distributed by Web advertising portals Lop.com and Xupiter.com, redirected browsers to their respective sites at every available opportunity. [CNET]

I was recently stricken by Xupiter, the lowest of scum-sucking buttwipes, but I was rescued by the outrageously great Spybot Search and Destroy. Once downloaded, SSD cleaned out my hard drive of all this crap post haste. Now I run the program once a week and it's nothing but smooth sailing and overboard to the leeches. Be sure to check regularly for updates and tip the guy who made this miracle (special thanks again to new Blogcritics Kurt Greiner who tipped me to Spybot Search and Destroy).