More than 300,000 people, including many in the US and UK, could lose internet access later as the FBI shuts off servers used by cyber thieves.
The FBI seized the servers in November 2011 during raids to break up a gang of criminals who used viruses to infect more than four million victims.
Victims' web searches were routed through the servers so they saw adverts that led to the gang being paid.
Many machines still harbour the gang's malicious code.
The gang racked up more than 14 million dollars, by hijacking web searches and forcing victims to see certain adverts.