Lenovo has been forced to remove hidden adware that it was shipping on its laptops and computers.
Security experts said the adware, which was named Superfish, was potentially compromising the security of the users' devices.
They added that the hidden software was also injecting adverts on to browsers using techniques similar to malware.
Lenovo faces questions about why and for how long it was pre-installed on machines and what data was collected.
In a statement the company said: "Lenovo removed Superfish from the preloads of new consumer systems in January 2015. At the same time Superfish disabled existing Lenovo machines in the market from activating Superfish.
"Superfish was preloaded on to a select number of consumer models only. Lenovo is thoroughly investigating all and any new concerns raised regarding Superfish."
Users began complaining about Superfish in Lenovo's online forum back in September, and in January Lenovo's forum administrator Mark Hopkins told users that due to "some issues" the company had "temporarily removed Superfish" from consumer systems until Superfish is available to provide a software build that addresses these issues.
He added that it had requested that Superfish issue an auto-update for "units already in the market".
Superfish was designed to help users find products by visually analysing images on the web to find the cheapest ones. Such adware is widely regarded in the industry as a form of malware because of the way it interacts with a person's laptop or PC.
Security expert Prof Alan Woodward said: "It is annoying. It is not acceptable. It pops up adverts that
you never asked for. It is like Google on steroids.
"This bit of software is particularly naughty. People have shown that it can basically intercept everything and it could be really misused."
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