Tens of thousands of U.S. internet users could be knocked offline on Monday when the FBI pulls the plug on domains related to "DNSChanger" malware.

Computers belonging to an estimated 64,000 users in the U.S., and an additional 200,000 users outside the U.S., are still infected with the "DNSChanger", despite repeated warnings in the news, email messages sent by ISPs and alerts posted by Google and Facebook.

The DNSChanger malware, which infected more than half a million machines worldwide at the height of its activity, redirected a victim's web browser to sites designated by the attackers, allowing them to earn more than $14 million in affiliate and referral fees.

IMAGES: What you should know about malware threat

In addition to redirecting the browsers of infected users, the malware also prevents infected machines from downloading operating system and antivirus security updates that could detect the malware and stop it from operating. When an infected user's machine tries to access a software update page, a pop-up message says the site is currently unavailable.

Users can determine if their computers are infected by visiting dns-ok.us in their browser. If an infection is reported there are a number of tools available to clean up their computers.