Dell Computer shut down its crucial internal networks dealing with sales processing for more than four hours yesterday when it became infected by the worst internet worm in 18 months.

But last night internet security experts were advising companies that the worst effects of the damaging "Slammer Worm" had passed following widespread publicity in the US and Europe.

The Slammer, which spreads through network connections rather than emails, causes servers to crash and internet traffic to become congested. It exploits a weakness in Microsoft's Windows 2000 SQL server database software, although it does not delete or otherwise touch data.

Most computers used by people at home or in businesses would not be directly affected by the worm because they are unlikely to use the SQL software. However, computer users yesterday trying to order goods or services over the Net may have experienced delays.

Dell's customer support and sales centre in Bray said it temporarily shut down its network to protect its business and customers.

"All backlogs of orders are currently being cleared. There was a minimal impact on the business as customers phoned orders through to the call centre rather than logging them online," it said.

Meanwhile, The Washington Post reported that the worm wrought havoc with Bank of America's ATM network.

The newspaper reported that customers were unable to process transactions for much of the weekend as the worm infiltrated the company's systems.

Mr David Bolger, technical director at internet security firm Entropy, said a software patch, freely available on the Microsoft website, would protect against the worm. - (Additional reporting, Reuters)