The agency asked computer users to do a quick check of their systems for the malware that could have taken over their machines more than a year ago.
The malware was designed to re-direct you away from trusted websites, towards spoof websites in a bid to steal financial and personal information, the FBI said.
Despite repeated alerts, the number of computers that probably are infected is more than 277,000 worldwide, down from about 360,000 in April.
The problem started when hackers across the globe ran an online advertising scam to take control of more than 570,000 infected computers around the world.
The FBI set up a safety net by installing two clean Internet servers to take over for the malicious servers to overcome the threat. Temporary system, however, will be shut down at 0401 GMT on July 9.
Experts said most victims don't even know their computers have been infected, although the malicious software probably has slowed their Web surfing and disabled their antivirus software.