A WORM is crawling through cyberspace hunting your computer. If your PC has slowed down sharply in the past few days or is constantly trying to reboot, the creature may already be slithering around your hard drive.

The latest terror of the internet, following on the heels of the Lovebug and the Melissa virus, is known by the names W32.Blaster.Worm, LoveSan or, more commonly, MS Blaster. MS refers to Microsoft, the American software giant that is the worm’s main target.

The worm — a type of computer virus — first surfaced late on Monday and has already caused significant damage to home and corporate computer users in Britain, America, Canada, Australia and New Zealand.

CERT, a government-funded internet security agency based at Carnegie Mellon University in Pittsburgh, Pennsylvania, estimated last week that more than 1m computers worldwide were already affected. Several American government agencies have reported network crashes as a result of the worm taking hold, and several variants of the original worm are already in circulation.

By the end of last week, British internet security firms were reporting a rash of calls from desperate clients whose systems had been attacked.

Clive Brown, director of Tricorp, an IT consultancy based in Kent, says: “In the past few days we have had a lot of calls. Earlier viruses like Lovebug and Melissa spread very quickly, but this one is particularly clever.”

Symantec, an IT security specialist based in California, estimates that Britain was second only to America in the number of computers affected, with 15% of all cases worldwide.

Unlike most viruses, which need an unwitting user to open an e-mail for a computer to be infected, worms roam the internet and creep in unnoticed while a PC is online.

MSBlaster uses a weakness in some Microsoft programs to attach itself to a host computer, where it begins its evil work. A number of different symptoms have been reported to Microsoft. As well as slowing up a PC’s normal functions, MS Blaster cunningly stops a PC from connecting normally to the internet, preventing the hapless owner from logging on to download a cure.

The worm also has a grander purpose. It is designed to use the computers under its control to launch a mass attack on Microsoft by continually logging on to the company servers that provide software updates to Microsoft users. D-day for the attack was yesterday.

Speaking on Thursday, Stuart Okin, chief security strategist at Microsoft in the UK, said: “As far as we know the worm sticks a file on your hard disk and in the process it crashes your system. On Saturday it will try to launch a ‘denial of service’ attack against Windows service sites. That’s the Trojan (horse) that we know about.”

There was a suggestion last week that the worm had other effects, though Okin would not be drawn on what these might be. Crashing systems is irritating, time-wasting but fairly benign. It would be much more dangerous if the worm were able to capture data from files or from key strokes, and post it on the internet.

Ironically, the worm springs from Microsoft’s own attempts to fix the original programming weakness, which has been known about for a month. In sending out a patch for the problem, Microsoft alerted malicious hackers, who devised a worm to take advantage of the chink in the armour.