Thousands of computers will be unable to access the Internet on Monday due to a virus released a year ago.

According to the Australian Communications and Media Authority (ACMA), up to 7,500 computers had been infected by the virus. The government agency has been working with international law officers in monitoring the virus which was discovered in November 2011.

The virus, known as "DNSChanger", allow the hackers to control the computers by changing its Internet access settings. The virus was disseminated by Estonian hackers as part of an online advertising scam. When the virus was discovered, it already affected 600,000 computers worldwide.

Until now, infected computers rely on dummy servers set up by the FBI to allow users to access the Internet. The FBI will turn off those servers at 2 p.m. Australian time next Monday.

Warnings about the virus spread across Facebook and Google. Moreover, Internet service providers have sent notices and the FBI has also set up a special Web site.

One of the problems faced by FBI is that if they turned off the malicious servers being used to control the computers, all the victims would lose their Internet service.

About 50 Fortune 500 companies are believed to be among thousands of server installations still infected by the virus. According to the FBI, the number of computers that probably are infected is more than 277,000 worldwide, down from about 360,000 in April. About 64,000 still-infected computers are probably in the U.S.

Most victims are unaware that their computers have been infected by the virus. The virus has the capability to slow the computers' online surfing and disabled their anti-virus software, making their machines more vulnerable to other problems.

However, popular social networking sites and Internet providers have gotten more involved, reaching out to computer users to warn of the problem. Facebook and Google created their own warning messages that showed up if someone using either sites appeared to have an infected computer.

Facebook users would get a message that says, ''Your computer or network might be infected,'' along with a link that users can click for more information. Google users, on the other hand, will get a similar message displayed at the top of a Google search results page.

Last March, ACMA put up Web sites that would allow Australian Internet users to test whether they had the virus. The agency urges Australians to visit dns-ok.gov.au and test their servers. Also, Internet users may visit http://www.dcwg.org, courtesy of the FBI, to check whether you have an infected server.

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