This week's MyDoom worm, one of the most destructive viruses ever, has already spawned a mutant version seeking to obstruct antivirus vendors' attempts to fight it.

Antivirus firm Panda Software said that MyDoom.B modifies the Windows host file, and redirects certain internet addresses including those of several antivirus vendors.

"This new variant is even more dangerous than its predecessor, as it is designed to prevent several antivirus programs from updating correctly," said Panda.

When users try to access their antivirus service, the internet browser shows an error message indicating that the page cannot not be found. This could prevent several antivirus programs from updating properly.

MyDoom.B is scheduled to launch a denial of service attack on Microsoft's website. The original version targeted The SCO Group.

The worm uses its own SMTP engine to send itself by email, usually bringing down the local network first. Like it predecessor, MyDoom.B also spreads via the Kazaa peer-to-peer network.

MyDoom.A has infected seven times more computers than Bugbear.B, the second virus most frequently detected by Panda.