The new outbreak, known as MyDoom.F, emerged late last week and has been gathering steam ever since.
The virus is programmed to infect personal computers and use them to unleash a crippling digital barrage known as a denial-of-service attack on select websites belonging to Microsoft Corp. and the Recording Industry Association of America (RIAA).
The attacks failed to bring down the sites, though access to the website for the RIAA was hampered slightly on Wednesday, security firms reported. The RIAA, a lobbying group for the music industry, has drawn the ire of computer users since it began suing American online song swappers last year.
While it was not spreading as fast as its MyDoom predecessors nor as rapidly as last week's Netsky.B outbreak, MyDoom.F is considered a growing risk as it deletes random Microsoft Word and Excel files, plus photos and movies stored on an infected computer.
"MyDoom.F has been picking up pace since Monday and Tuesday," said Mikko Hypponen, manager of Finnish anti-virus research firm F-Secure. "The disturbing thing is that it has a destructive payload. We haven't seen a destructive virus like this in a while," he said.
Computer viruses rarely destroy files these days. They have instead evolved over the years to turn unsecured computers into "zombie" machines capable of carrying out the virus writer's commands.
Typically, this army of commandeered machines is used to send out torrents of e-mail spam messages, unleash digital attacks on targeted websites and, in some cases, host websites that sell everything from vitamins to pornography.
"The disturbing thing is that it has a destructive payload. We haven't seen a destructive virus like this in a while"
manager of Finnish anti-virus research firm F-Secure
The first MyDoom worm surfaced in January and is considered the most virulent outbreak ever, infecting millions of computers around the globe.
Security firms were again advising computer users not to open mysterious-looking emails or click on their attachments if they are not certain of the sender's identity.
The latest outbreak arrives in e-mail in-boxes carrying a variety of subject header lines including: "Approved", "Your Credit Card" and "You use illegal File Sharing...Your IP was logged."