Important information stored on home computers could be wiped clean by an e-mail virus called Kama Sutra that is timed to strike today.
Computer engineers say the virus has been infecting hundreds of thousands of machines across the world since being identified on 16 January. It is being circulated as a pornographic attachment to e-mails and spreads by e-mailing itself to other addresses from an infected computer. On the third day of every month it deletes files.
The virus targets computers using Microsoft Windows but, unlike many other recent viruses, deletes many popular types of file, including spreadsheets and presentations.
A website associated with the virus estimated it has now reached more than 300,000 victims. Its impact is likely to be greatest on home computers because most businesses have current anti-virus protection.
The virus got its name because some of its variations refer to the Kama Sutra guide to sexual positions but is also known as "Nyxem-E, Grew.A", "MyWife" or "BlackWorm". It appears as an e-mail with subject lines such as "Hot Movie" or "Miss Lebanon 2006".
Affected files include Oracle, Word, Adobe Acrobat, Photshop, Excel and PowerPoint. Because so many of the applications are used in offices, experts believe the virus may be aimed at businesses.
Ken Dunham, from the US internet security firm VeriSign, said: "This attack is under way and will be activated unless people get virus removal tools.
"If you have opened an e-mail and your computer froze up, you should be very concerned."