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.
Global clean up
The gang racked up more than $14m (£9m) by hijacking web searches and forcing victims to see certain adverts. They managed to do this because their servers were taking over a key web function known as domain name look-up.
Domain names are the words humans use, such as bbc.co.uk, for websites. These are converted into the numerical values that computers use by consulting domain name servers (DNS).
When a person types a name into a browser address bar, often their computer will consult a DNS server to find out where that website resides online.
The gang infected computers with malware called DNS Changer because it altered where a PC went to convert domain names to numbers.
Since the FBI raids the gang's servers have been run by Californian company ISC.
Over the last few months, the FBI has worked with many ISPs and security firms to alert victims to the fact that their PC was infected with DNS Changer. Online tools are available that let people check if they are infected.
This has meant the original population of four million infected machines has been whittled down to just over 300,000, according to statistics gathered by the DNS Changer Working Group.
