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In addition to individual computer owners, about 50 Fortune 500 companies are still infected, Grasso said.

Both Facebook and Google created their own warning messages that showed up if someone using either site 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 got a similar message, displayed at the top of a Google search results page. It also provides information on correcting the problem.

To check whether a computer is infected, users can visit a website run by the group brought in by the FBI: http://www.dcwg.org.

The site includes links to respected commercial sites that will run a quick check on the computer, and it also lays out detailed instructions if users want to actually check the computer themselves.

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Associated Press writer Charmaine Noronha in Toronto contributed to this report.

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Online:

To check and clean computers: http://www.dcwg.org

Canadian websites: http://www.dns-ok.ca/

Comcast Warning: http://forums.comcast.com/t5/Security-and-Anti-Virus/DNS-Changer-Bot-FAQ/td-p/1215341

Google: http://googleonlinesecurity.blogspot.com/2012/05/notifying-users-affected-by-dnschanger.html

Facebook: http://www.facebook.com/notes/facebook-security/notifying-dnschanger-victims/10150833689760766