Microsoft has reached settlements with 11 computer retailers caught selling illegal software across the UK, the software company has announced.
The traders admitted to selling computers loaded with unlicensed copies of Microsoft's Windows operating system.
But Microsoft said the problem was not confined to the consumer market, with counterfeit software discovered in more than a third of corporate software reviews.
Microsoft said consumers and businesses can be exposed to damaging losses by rogue traders.
While the software company does offer replacement software to consumers duped by counterfeit products, businesses and victims of rogue traders are not covered.
Counterfeit or illegal software also exposes users to increase risk of attack from viruses and other malware, said Graham Arthur, anti-piracy lawyer at Microsoft.
"It is important to ensure you have genuine software that you are confident is not harmful or a threat to IT security," he said.
Microsoft has set up a website aimed at helping consumers and businesses to identify counterfeit or illegal software.
"Savvy buyers, who can tell the difference between the genuine article and illegal or fake software, can protect their businesses from being duped by unscrupulous retailers," said Arthur.