Experts say the virus, a variant of the MyDoom.A virus that knocked out another company's Web site on Sunday, was programmed to fire continuous volleys of debilitating data at Microsoft's site on Tuesday.

But there was no visible impact on the software giant's Web site, which barely flickered as the MyDoom.B Internet worm's trigger time of 13:09 GMT passed.

Microsoft had said on Monday it was taking a series of technical precautions to ward off any attack. The company declined to give any immediate comment on Tuesday.

MyDoom.B is a low-grade variant of the original MyDoom.A virus, the fastest-spreading e-mail contagion to ever hit the Internet, security experts said.
 
MyDoom.A has infected hundreds of thousands - and possibly over one million - PCs, generating a torrent of spam e-mails and crippling corporate e-mail servers, plus slowing traffic for some Internet service providers.
 
Biggest victim

The biggest victim of MyDoom.A was Utah-based computer software firm SCO Group. The week-old worm, also dubbed Novarg or Shimgapi, knocked the SCO site offline on Sunday with a barrage of data known as a denial of service attack.

SCO scrambled to launch an alternative site,thescogroup.com.

"It's now become less of a virus infection problem and more of an e-mail glut problem"

Graham Cluley,
Senior technology consultant, Sophos Plc

MyDoom.B, which was programmed to target both SCO and Microsoft, spread more slowly than its super-potent sibling and was never considered much of a threat, security experts have said.

"As far as MyDoom.B is concerned, you're more likely to see it in the headlines than in your e-mail in box," said Graham Cluley, senior technology consultant at Sophos Plc.

Still, security officials warned that MyDoom.A was still spreading rapidly despite the fact more computer users were fortifying their machines with a variety of free patches available from anti-virus vendors.

"It's now become less of a virus infection problem and more of an e-mail glut problem," Cluley said.