Anti-virus software companies say most computer users can expect to receive a flood of e-mails during the festive season.

"Just be vigilant," said Andrew Armstrong, general manager of anti-virus firm Trend Micro, "because you'll be getting e-mails with Christmas cards in or with attachments, and they could potentially be a virus.

"You need to make sure that if you receive anything unsolicited with an attachment and you don't recognise it, be very careful with it," he said. "Don't open it.

"You can get online checkers that will check your system for viruses and you should also look at putting some anti-virus software on to your home computer."

Mr Armstrong said that although most home users had heard about the dangers of viruses, most people thought that their machines would not become infected.

The warning comes as more and more companies are ditching the traditional Christmas card greeting to customers in favour of e-cards.

Around 40 British-based companies, including Marks & Spencer and Barclays, have stopped sending cards and will donate the savings to a charity for the homeless.

Cleverer and nastier

Accurate estimates of the economic damage that viruses inflict are hard to produce. However, in 2000, computer viruses caused US$17.1bn worth of damage worldwide, according to Trend Micro.

So far this year, the company estimates that viruses and worms have caused US$12bn of damage.

Anti-virus company MessageLabs has dubbed 2001 the "year of the virus".