A group of fast-spreading computer worms has infected thousands of machines, hitting media organisations, banks and other corporations through Microsoft's Windows 2000 software.

The Zotob worm, a malicious software programme spread over the internet, and its variants infected computers at the ABC and CNN television companies as well as The Associated Press, The New York Times, and The Financial Times forcing companies to take computers offline and take urgent safety measures.

Internet worms are spread with the ultimate aim of taking over computers, turning them into "zombies" that can be used to send spam messages or to host websites without the owners' knowledge. Any computer connected to the web is a potential target.

However, when a worm scans a network for other machines to infect it can overload systems, rendering computers useless and bringing companies to a standstill.

CNN broke into its regular programmes to report that computers at the news channel had been affected by a worm that caused them to restart repeatedly.

Other large multinationals reportedly hit included UPS, General Electric and Caterpillar. The virus also targeted banks, with at least two Canadian organisations, the Canadian Imperial Bank of Commerce and BMO Nesbitt Burns, suffering attacks.

The Zotob worm is similar to the Sasser worm which struck in May last year and the Blaster worm that crippled companies’ systems in 2003. However, it has spread more quickly than any other worm, leaving companies only a five-day window to protect machines.

Reports suggested one company a second could be being infected by the worms.

Patrick Runald, a senior anti-virus consultant with F-Secure, the computer security firm said he was surprised that companies had allowed what was in effect a re-run of previous attacks.

"Companies are aware that there are vulnerabilities in Windows which they should police. It seems most likely that at each of the infected companies employees have used laptops outside corporate firewalls that were infected and then linked back into the network."

The worms that have hit this week gain access to computers via a piece of software included in Windows 2000 called "Plug and Play", which is designed to aid adding new hardware and programmes to a computer.

Companies with large, networked computer systems were worst affected, David Perry, a security analyst at Trend Micro, a computer security company, said.

Mr Perry told The New York Times, which was temporarily forced to take some computers offline and install security patches, that there appeared to be seven different worms, with variants of those worms attacking the same vulnerabilities in the Windows 2000 software.

Last week, Microsoft released a "critical" patch to protect against the threat. The protective patches and instructions for cleaning infected systems are available on the company’s web site.

"We did not see a widespread or fast spread of this in the first 24 hours," said Debby Fry Wilson, director of Microsoft’s Security Response Centre.

"Over the last 24 hours, we’ve see variance, where other hackers will take the work and try to unleash a variant of the worm. So the worm continues to take on different forms."

McAfee, the computer security company, rated the worm a "high risk" threat. However, Mr Perry called the worm’s effects "small potatoes" in terms of potential damage.

Although Windows 2000 was most vulnerable, analysts said that companies running other versions of the software, such a Windows XP or 2003, could still be under threat.

The worm only needs to find one unprotected computer running Windows 2000 within a network to establish itself. The worm can then infect other versions of Windows.

"Think of a typical worm as a knife with one sharp edge," David Maynor of Internet Security Systems, told AP, one of the media organisations hit by the programme. "Think of this worm as a Swiss Army knife."

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