The clock is ticking on a dangerous computer virus programmed to delete millions of Word files stored on PCs when it reaches the end of its countdown on Friday.

Experts have warned that the Blackworm virus, which has also been named Blackmal, Nyxem, MyWife and Tearec, could destroy vast amounts of information when it is triggered, and could send unprotected businesses into chaos. The malicious software strikes against machines powered by Microsoft's near-ubiquitous Windows software.

According to SANS, the US-based internet security organisation, by January 26 around 300,000 computer systems, made up of potentially millions of individual PCs, had already been infected by the virus. The organisation said: "This worm is different and more serious than other worms for a number of reasons. In particular, it will overwrite a user's files on February 3."

The list of file-types that the virus will hit and delete includes the most commonly-used formats, including Word, Powerpoint and Excel documents.

However, experts have questioned whether Blackworm is only designed to carry out vandalism, or whether there is another, hidden motive.

Mikko Hyppönen, the chief research officer for F-Secure, a leading internet security company, recently told Times Online that internet attacks demanding money have became increasingly common in recent years, with organised gangs replacing lone adolescent hackers as the most dangerous source of malicious software. Most attacks are launched from infected home computers, which can be controlled through internet viruses released on to the net.

"Criminals usually target businesses such as online shops, which will lose money as soon as their website is offline, or sometimes we see companies targeting competitors, hoping to put them out of business," Mr Hyppönen said.

Windows users are now being urged to make sure their systems are not carrying the Blackworm virus, which is spread through e-mail messages, often claiming to include pornography. Subject lines reportedly used in the malicious e-mails include: "The Best Videoclip Ever", "Fw: SeX.mpg", "Miss Lebanon 2006" and "Kama Sutra pics".

If activated, the virus attempts to disarm a PCs security programmes and steal e-mail addresses to spread further. The virus also automatically downloads updates that could allow it to "mutate" and become even more dangerous.

Windows users are being advised to run scans for possible infections using up-to-date anti-virus software. Both businesses and individuals are advised to make back up files of important information.

SANS is running a page with updated technical news on Blackworm here.

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