Microsoft released a patch to protect against the worm late last year, while anti-virus software companies offer software to sniff it out and destroy it. Such tools can be expensive.

Technology research firm Gartner Inc estimates businesses will spend $13.6 billion on security software this year excluding costs for related labor, services and hardware. While some consumer anti-virus software packages are available for free, others run as high as $80 each.

Security experts suspect Conficker originated in the Ukraine, based on its code. The FBI is working to shut it down but a spokesman declined to comment on its investigation.

"The public is once again reminded to employ strong security measures on their computers," said Shawn Henry, assistant director of the Federal Bureau of Investigation's cybercrimes division.

Independent security firms such as McAfee, Symantec Corp and Trend Micro Inc say they will closely monitor cyberspace on Wednesday to see how the worm mutates but will also watch closely over coming weeks as the hype fades.

"I don't expect much to happen on April 1st. That's the one day I would not do it. That's the one day everybody is watching for something to drop," said Joe Stewart, director of malware research at SecureWorks. "It's just another small step in whatever the end game is."

Reuters