In three weeks, the FBI could knock millions of infected systems offline by disabling some DNS servers. In November, Estonian authorities arrested six men suspected of using "DNSChanger Trojan" malware to redirect victims to malicious websites and block them from genuine security sites that might've removed the infection. According to a report by Brian Krebs, the Trojan was on over four million computers in more than 100 countries, including 500,000 in the US -- and not just home users, either. The malware is reportedly still present on computers at half of the Fortune 500 companies and at approximately half of all federal agencies.

Come March 8, the court order allowing the feds to replace the bogus DNS servers will expire. Unless that's extended, the surrogate servers will be unplugged and millions still plagued by DNSChanger will lose Web access. Even the order is prolonged, experts worry the cleanup will take years as with Conficker.

Checking your system for DNSChanger is relatively simple. You can use ipconfig /all in the Windows command prompt to get your DNS information, which can be compared against the numbers here. The DNSChanger Working Group also provides a list of free resources that'll help you clean your machine(s).