A virus that may have been sitting in hundreds of thousands of computers across the world is expected to wreak havoc on July 9, the FBI has warned in an Associated Press report.

The FBI made the announcement as it prepares to turn off a server system that its investigators installed in November, after they caught an Estonian crime ring that was operating a massive advertising scam on the Internet.

Once the particular server system was shut down, an undetermined hundreds of thousands of computers affected by the virus will simultaneously lose access to the Internet.

The virus, called "DNS Changer," operates as its name suggests - it controls the domain name system, the system that makes Internet users reach the Web site they want to see.

The virus is Trojan in nature, meaning it makes itself look like a useful part of the computer system, but once installed, it causes considerable damage.

"DNS Changer" redirects the users to sites that are controlled by the Estonian crime ring, effectively grabbing control and making users unable to use the Internet. In turn, traffic to the sites would generate millions of advertising revenue, if the crime ring was still operational.

The crime ring had been running since 2007. They have reportedly earned US$14 million in commission from redirecting Web sites. When the FBI and Estonian authorities shut them down, "DNS Changer" had already seized control of about half a million computers.

Internet users are advised to check if they are victims of the virus, and then remove it from their computer.

Check your computer: http://www.dcwg.org/detect/