Computer users have been advised to ensure their anti-virus software is up to date as the "black worm" virus is expected to strike today.
Microsoft issued an advisory warning customers about the worm, which affects most versions of its Windows operating system.
The virus will corrupt documents using the most common file types, including ".doc," ".pdf," and ".zip."
Hundreds of thousands of machines are already believed to be infected, mostly in India, Peru, Turkey and Italy.
The worm, also known as CME-24, BlackWorm, Mywife.E or a number of other monikers, even tries to disable anti-virus software that is out of date. Users should make sure their software is turned on and has the latest definitions, generally available from the software vendor's website.
Users should be safe if they have the latest anti-virus software or if their computers are set with limited privileges, a common setting in larger organisations.
Users should check the date on the computer. The worm hits the third of every month, so if the computer's local calendar settings are off, files may be destroyed sooner or later, even if the computer is never turned on today.