BT broadband customers whose computers have been hijacked by spammers will now be contacted by the company and shown how to clean up their computers.

The telco has now implemented what it says is the world’s first fully-automated ‘spam buster’ system which will be able to track down and tackle professional spammers in the UK and botnet-infected customers on the BT broadband network.

Consumer PCs that have been infected by spyware and Trojans so that they can be remotely controlled by hackers to send spam or launch denial of service or other attacks are often called zombies.

The problem for internet service providers (ISPs) in the UK is when these zombies are linked together, they form powerful networks called botnets.

Anti-spam organisation Spamhaus estimates that 80 per cent of all spam originates from compromised PCs and the latest research from Webroot shows that in the UK an estimated 89 per cent of consumer computers are infected with around 30 individual spyware infections.

The new spam-detection system selected by BT – Content Forensics from StreamShield Networks – scans millions of emails a day, providing BT with detailed reports on the location and size of spam-related problems originating from the BT network.

As well as eating up bandwidth, innocent internet users often find that they have been blacklisted by other ISPs as spammers. This can happen even if their PC is not infected because of the way IP addresses are allocated.

An IP address is a unique number, like a street number, that allows devices to identify and communicate with each other on a computer network. When consumers log online they are usually given what is called a dynamic IP address.

This is assigned from a small pool of addresses to a larger number of customers. Because the consumer will get a different IP address from this pool each time they log on, an ISP tends to blacklist the whole pool even if there is only one person sending spam.

The new spam-buster system means BT's Customer Security team can now not only take immediate action against professional spam operators but also focus more effort on contacting and helping customers rid their PCs of botnet infections.

A BT representative said it will call customers it identifies as part of a botnet and talk them through how to clean up their PC and protect it against future attacks.