The most up-to-date anti-virus software will spot CIH

The virus, known as CIH, is capable of lurking in a computer until 26 April and then causing havoc to the data stored there.

It is also called the Chernobyl virus because it is timed to go off on the anniversary of the Russian nuclear accident, one of technology's worst disasters.

Most network users do not have much to worry about, says the BBC's Chris Nuttall

CIH is a potentially deadly virus for computers. As well as deleting files stored on the hard drive, it also attempts to corrupt the heart of any computer - the flash BIOS.

This can cause complete loss of data, and possibly render the computer unusable.

Risk to W95/W98 users

Users of DOS, Windows, Windows NT or Macintosh users are not at risk from CIH. It only replicates and activates under Windows 95 and Windows 98.

The Melissa scare has alerted users to CIH, says Mikko Hypponen of Data Fellows Software

The virus is designed to hide from view by inserting itself into empty coding slots on a computer's software utilities.

But most up-to-date anti-virus software will spot it. There are various free checking programs that can be downloaded from the Internet to scan a computer for the bug.

Common virus

The virus was first located in Taiwan in early June 1998 and then spread to the wider world.

It has been among the 10 most common viruses for several months, spreading very quickly as it has been distributed through pirated software and the Internet.

It is not widespread in Europe or the US, but very common on computers in Asia. CIH is a program, so it must be activated before it can take effect.

"There would be no way of knowing that you had downloaded this virus and run it, it's completely invisible to the user," said virus expert Mikko Hypponen of software company DataFellows.

The CIH virus is far more dangerous to individual computers than Melissa, the much-publicised macro bug that spread relatively benign problems far and wide on the Internet last month.

Melissa was mainly spread by e-mail, and relied on people opening a document attached to e-mail messages.