A new internet virus spreading rapidly around the world may already have infected millions of computers.
Installing a Microsoft security patch prevents infection
Sasser, unlike a virus which travels through e-mails and attachments, spreads directly from the internet.
It attacks recent versions of Microsoft's Windows causing the computer to shut down.
The worm exploits a security flaw, but this can be prevented with a Microsoft patch.
It typically shuts down the computer then automatically re-boots it and repeats this process several times, but is not thought to cause lasting damage.
Alfred Huger, senior director of engineering at Symantec, based in California, said the worm "breaks into your computer and then attempts to
break into others".
Affects Windows 2000
Windows Server 2003
Microsoft patch prevents infection
"It chooses its victims randomly," he added.
Mr Huger said someone had started the virus deliberately.
"Of that much we're sure."
Mikko Hyppoenen, an anti-virus expert at Finnish internet security company F-Secure, said: "It was probably some hobbyist, a teenager who has the skills
and wants to show off.
"We don't know how big this is going to be - but we expect things to get much worse when people bring their laptops in to the office after the weekend," he added.
Graham Cluley, senior technology consultant for Oxfordshire-base software security company Sophos, said home users were especially vulnerable.
"They are often not running the latest anti-virus protection, haven't downloaded the latest security patches from Microsoft, and may not
be running a personal firewall," he added.