The MyDoom worm, one of the most virulent yet detected, has already spawned a mutant version that tries to obstruct attempts to fight it.

Antivirus firm Panda Software said that MyDoom.B prevents computers from contacting certain specified internet addresses, including those of several anti-virus companies.

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

When users try to access their anti-virus 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.

Panda is offering free tools to clean up the effects of both MyDoom variants. Concerned users should first read the advice on Panda's website here.