The US Federal Bureau of Investigation (FBI) has warned that thousands of people could lose internet access in July unless they run an anti-virus scan.
The problem is caused by some malware called DNSChanger, which dates back to 2007, and has infected millions of computers around the world.
The malware is understood to have infected PCs, redirecting their web browsers to rogue DNS servers to help criminals make money through ad click fraud.
The scam netted the criminals $14 million before the gang was disbanded following the arrest of six Estonians.
Had the FBI simply dissolved the criminals' infrastructure and shut down the rogue servers, users would have lost internet access there and then. However, on the evening of the raid, a consultant was hired to install two clean servers to take the place of the impounded ones.
These servers were due to shut down on 8 March, but a federal judge extended the deadline to 9 July.
Meanwhile, the FBI has been trying to contact users whose devices are still infected to direct them to the DNS Changer Working Group (DCWG) website, where the problem can be fixed.
The FBI said the fix was only there to buy "additional time for victims to clean affected computers and restore their normal DNS settings."
Users have until 9 July to check if their computers are still infected. After which the website www.dcwg.org will shut down.
