CoolWebSearch, adware that generates more than $300 million a year for its maker, is the "Ebola" of adware, and easily the most significant spyware threat on the Internet, an anti-spyware security firm said Wednesday.

CoolWebSearch, which comes in multiple forms, can hijack Web search errors, usurp the browser''s home page, and modify other Internet Explorer settings. Recent variants have taken to exploiting vulnerabilities in IE, such as those in the HTML Help system, to install on PCs.

"It''s only purpose is to get on a PC, and stay on that PC, even at the cost of killing that machine," said Richard Stiennon, the vice president of threat research for Boulder, Colo.-based Webroot, which publishes the Spy Sweeper line of anti-spyware software.

According to Webroot, nearly half of the PCs it''s audited for spyware or adware are infected with CoolWebSearch.

"It''s the Ebola of the Internet," said Stiennon. "It''s so malicious that it tends to break the ability of a machine to browse effectively, and therefore limits the number of ads and click-throughs that can be generated. Like Ebola, it kills its host before it can be productive."

Webroot''s newest Top 10 list -- it releases a list of the ten most significant spyware/adware threats every quarter -- is based on the free spyware audits it conducts from its own Web site, and those it runs in cooperation with EarthLink, the Atlanta-based ISP.

"We rank programs on both prevalence and perniciousness," said Stiennon.

Second on Webroot''s list is Gator/GAIN, adware that may display banners ads based on Web surfing habits. Gator is a long-time adware package that often gets on systems because it''s bundled with free software, most notably the P2P file-sharing program Kazaa. By the SpyAudit scanning results, Gator/GAIN is on about 15 percent of all machines.

"If we take the leap and assume that the sample is representative of the Internet in total, we can estimate how many machines have Gator," said Stiennon. His best guess: 38.4 million PCs. Others on Webroot''s list include (in descending order), 180search Assistant, ISTbar/Aupdate, Transponder, Internet Optimizer, BlazeFind, Hot as Hell, Advance Keylogger, and TIBS Dialer. Most are adware in composition -- not that that means they''re benign; they typically hijack search errors and re-direct them to another site, and/or blitz the PC with endless popups -- but some are true spyware.

"We''re finding keyloggers on about 15 percent of the machines audited," said Stiennon, "and Advanced Keylogger is the most prevalent right now. It''s on relatively few machines -- about 9,000 that we''ve found -- but a keylogger on that many PCs is a scary concept in and of itself.

"Spyware writers are continuing to innovate and find new, more deviant ways to infiltrate systems," said Stiennon. "The increased presence of hijackers, dialers, and keyloggers demonstrates that the new trend for these threats is to go straight for the jugular."

Spyware/adware writers are doing that for one reason: money.

Stiennon, who has analyzed the spyware/adware economy, has come up an average cash flow per "customer installation" per year of $2.40. For each system infected, then, he estimates that the adware author generates $2.40 annually in pop-up fees, redirect fees, and other charges.

His cash-flow projection for the creator of CoolWebSearch -- which using his formula may be on more than 127 million machines worldwide -- is thus $306 million. The company behind Gator/GAIN -- the Redwood City, Calif.-based Claria -- is bringing in around $92 million a year, while 180search Assistant is raking in $86 million.

"These guys make spammers look like two-bit back alley operations," said Stiennon. "No wonder there''s a gold rush to get in on this."

And no wonder some adware firms are pushing anti-spyware vendors to "de-list" them from their detection and deletion scanners.

The most recent such move was by Computer Associates, which sells the PestPatrol anti-spyware line after acquiring the company in 2004. Last week, CA removed all Claria products -- including Gator/GAIN -- from its database under its Vendor Appeal program.

CA has been criticized in the past for de-listing software other anti-spyware vendors continue to list as malicious, and even Microsoft has backed down in at least one instance.

"One reason Webroot publishes the Top 10 list," said Stiennon, "is to help provide an idea of the scope of the whole spyware and adware issue, so that going forward, as the discussion of adware heats up and definition battles with the vendors begin, people will have some basic information about the extent of the problem."