Microsoft said yesterday that its websites were "fully available" despite fears of a catastrophic computer virus attack.

The Mydoom virus, which has blighted home computers and shut down the website of one software company, was programmed to attack Microsoft yesterday.

But early indications were that the bug had been unable to penetrate the computer giant's security systems.

It had been feared that the virus would shut down Microsoft websites, creating havoc for millions of computer users worldwide.

Wiping out such a giant is seen as the holy grail for computer "vandals".

Despite their early successes in fending off the attack, Microsoft experts were remaining vigilant. A Microsoft spokeswoman said: "Microsoft's web properties remain fully available to customers.

"We continue to be focused on ensuring that our websites are available to customers and provide them with critical resources, including the latest security information."

The spokeswoman added that the company was "working to stamp out the criminal behaviour behind these malicious attacks" by issuing a $250,000 (£136,000) reward for information leading to a conviction of a virus distributor.

The virus works by getting infected computers to send hundreds of thousands of requests for a website per minute, crippling the particular site.

The virus targeting Microsoft is a variant of the Mydoom virus, called Mydoom B.

An earlier version, Mydoom A, has shut down the website of Utah software company SCO Group.

But security experts said that the Mydoom B variant was spreading much less quickly than Mydoom A.

Microsoft has set up an alternative website for people whose computers are infected with the virus. The latest reward is the third so far under a $5m reward programme Microsoft announced in November to help authorities catch the authors of internet infections.

SCO Group had previously offered a reward of $250,000 for information leading to the arrest and conviction of the creator of Mydoom A.