Virgin Media is the first UK internet service provider to warn customers their PCs are infected with a virus.
Virgin Media sent letters to 1,500 customers, warning them they had been targeted by the SpyEye trojan. The SpyEye malware steals online banking credentials.
Virgin Media is providing information on how disinfect PCs with the SpyEye trojan and offering customers a PC Healthcheck utility.
"With increasing numbers of people getting online and a nation reliant on broadband, consumers are looking for more from their service providers to give them the confidence their online world is safe," said Jon James, executive director of broadband at Virgin Media.
The SpyEye trojan collects personal and banking information and represents a high level of threat to infected users, Virgin Media said.
Security firm Trusteer has revealed that a variant of SpyEye is targeting online customers of Germany's second largest airline, Air Berlin, and travel services firm AirPlus.
Virgin Media said it had been alerted to the malware infection by the Serious and Organised Crime Agency. The agency identified Virgin Media customers as targets while investigating criminal botnets.
Lee Miles, head of cyber at Serious and Organised Crime Agency (Soca), said the agency works with a range of private sector partners to help prevent cyber criminals from exploiting legitimate businesses and their customers.
"We welcome steps taken within industry to utilise the information and resources provided by law enforcement and raise awareness of online safety," said Lee Miles.
But it is equally important for consumers to help protect their finances and personal information by ensuring their computers are equipped with up-to-date security software, said Soca's head of cyber.
Virgin Media revealed that around a quarter of customers contacting the company's support services have not installed any type of computer protection or are failing to keep their security software up to date.
Miles said UK users of online services should make use of the free information on improving their defences available from GetSafeOnline.