By BBC News Online's Damian Carrington

An analysis of the most common computer viruses of 1999 shows that although the threat of new self-propagating viruses is growing, older viruses are still very common.

One boot sector virus, Form, is nearly a decade old but still appears in the top ten. The table was compiled by anti-virus software firm Sophos, based on thousands of calls for help to the company.

The three self-propagating viruses were Melissa, ExploreZip and Ska-Happy99 which forward themselves by hijacking a computer's email program. This means that instead of taking months to spread into the wild, these viruses have the potential to attack globally within days.

However, Graham Cluley, senior technology consultant for Sophos, believes that old viruses still pose a major threat: "Some viruses become so common, they will never become extinct - they will always lurk on a floppy disk in someone's drawer.

"Also, people may be aware of the latest scare but not the background threat. It's difficult to get people excited about old threats."

Spreading out

The most reported virus in 1999 was a macro virus called Laroux and was first detected in early 1996. Unusually for a widespread macro virus, Laroux infects Excel spreadsheets rather than a Word document.

"It may be that people are getting quite cautious about opening documents, as they may have been hit by that before, but are not so used to the threat of spreadsheets," says Mr Cluley.

According to Mr Cluley, the key to long-lived viruses is being virtually invisible. "Viruses which jump up and down with very destructive payloads draw attention to themselves and effectively kill themselves off, like lemmings.

"Form does nothing, it just spreads, although it still causes damage by using up system resources."

Silent but deadly

Whilst having your hard disk wiped by a virus may seem the computer equivalent of Armageddon, many companies and individuals keep back-up copies of information. Some of the most damaging viruses are not destructive at all, says Mr Cluley.

"Some, like Melissa, can forward documents to e-mail addresses stored on your computer - highly confidential information has leaked from companies in this way," he says.

And "data diddler" viruses exist which make subtle changes to data in a spread sheet. "If those are your company results, it could be very embarrassing," he adds.

The year 2000 will see hoax viruses - email warnings of non-existent viruses - continue to cause enormous problems believes Mr Cluley. "In a way they are far more damaging than real viruses as they set off e-mail hurricanes and you can't disinfect a hoax.

"We had far more people seeking information on a hoax about a game involving Santa and his elves than any real virus."

Finally, Mr Cluley and other anti-virus experts are awaiting the sentencing in February of David L Smith, who pleaded guilty to distributing the Melissa macro virus and admitted causing more than $80m damage to North American companies.

"We are rather hoping that, depending on what the sentence is, it may send out a message to virus authors that this isn't cool and the authorities are prepared to pursue you."