The worm - which attacks computers running the Windows operating system - spreads by mass-mailing itself from infected computers. It may also infiltrate folders shared by two or more computers.

Once executed, the worm creates new files in the Windows systems folder.

Fortunately, Zafi.b is more of a nuisance than a serious attack, otherwise systems would be crashing all over the world. It was merely slowing down e-mail systems, said Michael Gazeley, of Network Box Corporation, who estimated the worm first arrived at the weekend.

The corporation is a local computer security company that monitors computer attacks.

'The Zafi.b worm is currently attacking Network Box-protected sites at a rate of more than one attack per minute. It comes in through e-mail or through peer-to-peer connections,' Mr Gazeley said.

The attacks were so effective because anti-virus companies were not responding fast enough, he said. 'The anti-virus companies did not get anti-virus signatures onto computers in time, otherwise it would not be spreading so fast.'

'We had a lot of customers protected, but they were getting several thousand additional e-mails every day.

'It underlines once more just how many systems out there are not adequately protected. This particular worm is not much more than a nuisance, but it could have been far worse,' he said.

The worm was discovered at the weekend and reports said it might have originated in Hungary.

Mr Gazeley said he was discovering about 1,000 new viruses every month.