Microsoft is better placed to avoid an impending attack from the MyDoom worm, the spam e-mail virus that has infected hundreds of thousands of personal computers worldwide and brought down the Web site of a small software provider, experts say.

A more robust Web site, better technological resources and the more limited spread of the MyDoom variant targeting Microsoft's home page will make it less likely that the worm will succeed in Tuesday's pre-planned Web attack.

Infected machines are instructed to flood SCO and Microsoft's Web sites with requests for information in an attack called a distributed denial of service, and also give potential attackers unauthorised access to compromised PCs.

Separately, Microsoft issued a patch on Monday for its Internet Explorer Web browser, which was discovered to have a critical vulnerability that would let attackers take control of a user's computer.

The week-old worm, also dubbed Novarg or Shimgapi, continues to spread and has already shut down the Web site of the SCO Group Inc., a small software developer that has become the target of Linux enthusiasts.

The worm is named after some of the words found in its files.

The next target, by a variant called MyDoom.B, is Microsoft's Web site, but the world's largest software maker said it was prepared for the onslaught, which forced SCO to shut down its Web site and offer customers a new Internet address.

"There are things that we can do .. .depending on what kind of traffic we see," said Mike Reavey, a security program manager at Microsoft's security response centre.

Security experts said that Microsoft appeared ready to withstand the onslaught scheduled for Tuesday, because of the resources the muscular software company has at its disposal, as well as the fact that MyDoom.B has infected far fewer machines than the original MyDoom outbreak.

Both software companies offered separate $250,000 bounties for information leading to the conviction of the virus writer and are working with various law enforcement authorities to flush out the culprit.

The worm, which is designed to entice recipients of a official-looking message to open an attachment, spreads as spam and is estimated to have infected as many as a million PCs worldwide.

Adding to user's troubles was an unrelated announcement by Microsoft of a flaw in its software that could turn PCs into drones for attackers and two other flaws that could potentially allow attackers to save files on a person's computer and display false Web pages.

Microsoft said it released the patch eight days ahead of a regularly scheduled security software update in order to warn customers of the threat.

"We decided to release the patch based on the severity of the threat," Microsoft's Reavey said.

Users having difficulty reaching Microsoft's home page http://www.microsoft.com, the gateway for security patches, product information and other services, can go to http://information.microsoft.com, Microsoft said.

SCO, which saw its site, (http://www.sco.com) brought down on Sunday, is advising visitors to visit (http://www.thescogroup.com).