On Dec. 22, an Internet investigator got a tip that child pornography was being housed on an adult Web site. When he visited the site to verify the information, he didn't find any illegal images. But what he did find was a Trojan horse that disabled the ActiveX security controls on his browser and took control of it.

"I heard my hard drive churning and clicked on my task manager and saw three executable programs were installing themselves," says Chris Brandon of Brandon Internet Services. "I knew I was in trouble when I couldn't get my task manager to cancel the programs."

By the time he checked his registry, the Trojan had installed dozens of programs that replaced the default Web page with its own, and loaded its own IP addresses in his favorite places, short cuts and safe zones. When he tried to erase the programs and reboot the machine, the virus reinstalled.

This program is a perfect example of spyware gone amok.

It installed itself by taking advantage of a vulnerability in Internet Explorer 4.x and 5.x that lets an unsigned applet to create and use ActiveX controls. Then it hijacked Brandon's browser, a term called "Web-jacking." But it could have been worse. Some variants evoke dialers to call up 1-900 numbers if the victim is using telephone dialup for Internet access.

"We're seeing more of this type of virus activity in recent months," says Ken Dunham, director of malicious code for iDefense, a security intelligence firm in Reston, Va. "Trojans promote going to certain pornography sites and other sites they affiliate with because they get money for the clicks from advertisers. They terminate regedit.exe (registry editor), and they can be very difficult to remove."

Anti-spyware vendor PestPatrol Inc. reports staggering growth over the past few months of the virus that Symantec Corp. dubbed Trojan.Norio. And at least 24 variants of the virus now exist in the wild, according to the anti-spyware site Spywareinfo.com.

Each variant is designed to do something different. One variant changes your customized search settings to allhyperlinks.com, for example. Another variant redirects all searches through a bogus site called Coolwebsearch.com. Another redirects Verisign Inc.'s Site Finder to a fraudulent Site Finder site. Another evokes the auto-dialer. And so on.

Expect these types of Trojan viruses to be used for even more malicious purposes, such as the culling of credit cards and passwords, Dunham says.

"In the case of the Norio Trojan, it changes the registry and the host file," he says. "You type in a name like Microsoft.com, it will redirect you to a site they want you to go to. You could make it redirect you to a fake Citibank.com Web site and get you to fill in sensitive information."

Brandon removed the malicious code by using Spywareinfo's remediation kit called CWSweep. (PestPatrol also provides a removal kit.) He's since been tracking down the IP addresses and domain names that the virus loaded into his registry. Many of the domain names are a variation of Coolwebsearch.com.

"I want to find the people responsible for this, the affiliates in collusion with this, and turn them into Microsoft for that bounty it promises on virus writers," he says.

With the IP addresses and Web site names so easy to find, you'd think tracking the virus writers would be easy for someone with Internet tracking skills. But most of the IP addresses Brandon's investigated led to bogus hosting providers and anonymized administrative contacts. Meanwhile, the PestPatrol report on the virus lists an address for Coolwebsearch.com, the originator of the virus, to be in Natick, Mass.