The rapid growth of broadband adoption has increased the number of compromised Windows PCs in the UK, experts warn.

During the first half of 2005, nearly a third of the world's bot-infected computers, called "zombies," were located in the UK, according to Symantec.

Symantec estimates that there are between 1 million and 2 million computers worldwide infected with this bot software, which allows a system to be surreptitiously remote-controlled by hackers, said Dean Turner, senior manager of the Symantec Security Response team.

Spirit of the age

"Bot networks are valuable for a couple of reasons: One, because they allow for extremely rapid propagation, and two, because they provide a relatively high level of anonymity for providing attacks," Turner said.

On average more than 10,000 bot-infected machines were active each day during the first six months of the year, an increase of more than 140 per cent over the preceding six months, according to Turner.

These networks of zombie computers have become a weapon of choice for spammers and phishers as well as attackers looking to swamp a victim's server with a flood of unwanted data, a technique called a distributed denial of service attack.

Winsford's weasels

In its biennial Internet Security Threat report, set to be published Monday, Symantec found that London and Winsford, England, were the top two bot-infected cities worldwide, with 8 per cent and 5 per cent of the world's infected computers, respectively. Seoul came in at third place, with 4 per cent.

The US and China were the second-largest and third-largest providers of bot-infected systems, with 19 per cent and 7 per cent, respectively.

London is considered one of the largest hub cities on the Internet, with a total of 1.1T bps (bits per second) of international bandwidth available in the city, according to research company Telegeography.

Use of DSL (digital subscriber line) broadband has been growing rapidly throughout the UK. Earlier this year BT announced that it had more than doubled its DSL connections over the previous year's number, reaching a total of 5 million lines 12 months earlier than expected.