The new virus, named Zindos, piggybacks on the success of the recent MyDoom.O virus. Zindos compromises computers infected with the earlier virus and is programmed to visit the Microsoft home page every 50 milliseconds.
Earlier this week Google, Lycos, Altavista and Yahoo all struggled to cope with the MyDoom.O virus, which created endless loops of requests directed at the targeted Web sites.
So far, the Microsoft web site appears to be unaffected by the attack. In a statement released on Wednesday, Microsoft said it had "taken steps" to ensure its Web site remains online.
Zindos is spread from infected computer to infected computer, via a "back-door" in the MyDoom.O virus. That opening allows Zindos to bypass other virus protections and infect a computer. It then uses the comprised machine to attack the Microsoft site and search for other computers to infect.
The virus affects Windows systems but not computers running the Linux or Apple operating systems.
Computers users who suspect they might have a comprimised machine can download a free removal tool from Symantec's web site.