Sasser, a new computer "worm" that targets PCs directly through a weakness in Microsoft's Windows software, disrupted systems across the world yesterday as it spread via the internet. The worm is considered one of the most serious since the "Blaster worm" attacked millions of Windows PCs last August. Some security experts believe it has already infected more than 1m users. Unlike other worm-type viruses, PC users do not have to open e-mail attachments to trigger it. Instead, it actively seeks PCs connected to the internet that run Microsoft 2000 or XP, entering through a known vulnerability in the software.

Once infected, the worm makes a computer automatically shut down and re-boot. It is thought to be more of a threat to home users and small businesses, since corporate firewalls protect most business users. Microsoft has confirmed that the Sasser worm and a number of variants are circulating on the internet and the software company has rated the vulnerability as critical.