A woman looks at her email on a computer screen in Washington on November 23, 2010. Americans will take a break from the office over the Thanksgiving and Christmas holidays but most won't stop checking their work emails, according to a survey released on Tuesday. The Harris Interactive survey conducted for Xobni, an email software firm, found that 59 percent of employed American adults will consult work emails over the holidays. Fifty-five percent will check work emails at least once a day and 28 percent will do so multiple times throughout the day, the survey found. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images) File (Credit: Getty Images)

WASHINGTON (CBS/AP) – For computer users, a few mouse clicks could mean the difference between staying online and losing Internet connections next week.

Unknown to most of them, their problem began when international hackers ran an online advertising scam to take control of infected computers around the world. In a highly unusual response, the FBI set up a safety net months ago using government computers to prevent Internet disruptions for those infected users. But that system is to be shut down.

The FBI is encouraging users to visit a website run by its security partner, www.dcwg.org, that will inform them whether they’re infected and explain how to fix the problem. After July 9, infected users won’t be able to connect to the Internet.

Most victims don’t even know their computers have been infected, although the malicious software probably has slowed their web surfing and disabled their anti-virus software, making their machines more vulnerable to other problems.

Last November, the FBI and other authorities were preparing to take down a hacker ring that had been running an Internet ad scam on a massive network of infected computers.

“We started to realize that we might have a little bit of a problem on our hands because … if we just pulled the plug on their criminal infrastructure and threw everybody in jail, the victims of this were going to be without Internet service,” said Tom Grasso, an FBI supervisory special agent. “The average user would open up Internet Explorer and get ‘page not found’ and think the Internet is broken.”

On the night of the arrests, the agency brought in Paul Vixie, chairman and founder of Internet Systems Consortium, to install two Internet servers to take the place of the truckload of impounded rogue servers that infected computers were using.

Federal officials planned to keep their servers online until March, giving everyone opportunity to clean their computers. But it wasn’t enough time. A federal judge in New York extended the deadline until July.

Now, said Grasso, “the full court press is on to get people to address this problem.” And it’s up to computer users to check their PCs.

This is what happened: Hackers infected a network of probably more than 570,000 computers worldwide. They took advantage of vulnerabilities in the Microsoft Windows operating system to install malicious software on the victim computers. This turned off anti-virus updates and changed the way the computers reconcile website addresses behind the scenes on the Internet’s domain name system.

Victim computers were reprogrammed to use rogue DNS servers owned by the attackers, which allowed the attackers to redirect computers to fraudulent versions of any website.

The hackers earned profits from ads that appeared on websites that victims were tricked into visiting. The scam netted the hackers at least $14 million, according to the FBI. It also made thousands of computers reliant on the rogue servers for their Internet browsing.

When the FBI and others arrested six Estonians in November, the agency replaced the rogue servers with Vixie’s clean ones. Installing and running the two substitute servers for eight months is costing the federal government about $87,000.

Vixie said most of the victims are probably individual home users, rather than corporations that have technology staffs who routinely check the computers.