The Mydoom e-mail worm brought the website of software firm SCO to a complete standstill over the weekend in a massive denial-of-service attack. Microsoft is bracing itself for a similar attack in the next 24 hours by the worm - the fastest-spreading virus seen so far.
SCO, a target for Linux supporters because of its copyright claims to the open-source software, said the virus was "overwhelming the internet with requests to www.sco.com", adding that the attack could last until 12 February.
Both SCO and Microsoft have each offered £140,000 rewards each for help in catching the worm's author.
The first version of the virus, Mydoom.A - also known as Novarg or Shimgapi - emerged last Monday in the form of a spam e-mail message which contained a well-disguised virus.
Mydoom.B, which is targeting Microsoft, has a larger payload and could prove more damaging, according to security experts
Ken Dunham, director of malicious code at security consulting company iDefense, said the Mydoom.b worm modifies the standard hosts file in a Windows folder that can block access to 65 websites, most of which are anti-virus websites, in an attempt to block users from downloading anti-virus solutions and data.
"This 'b' variant of Mydoom is worse than Mydoom.a," he said. "An attack on the Microsoft.com website could cause a significant disruption of services for users worldwide."
Microsoft said it is working with the FBI, the US Secret Service and Interpol in investigating the Mydoom.B worm, the release of which the company described as a "criminal attack".