Viruses once knocked out whole workforces for a week or two, but 21st century viruses delivered via the internet are rather more serious than snuffles and sneezes and can wipe out whole companies in just a few seconds.

Such is the warning from Business Link Berkshire and Wiltshire (BLBW), which claims that despite 90 per cent of business using anti-virus software, well over half of all firms in the region were caught out last year by virus attack.

Indeed, in just the first three months of 2004, the viruses MyDoom, Netsky and Bagle infected more computer systems than ever before, resulting in loss of sales and occasionally, the loss of an entire business.

According to Andy Poulton, e-business adviser for BLBW: "By trying to infect computers, the viruses this year have two primary aims.

"Firstly they look to replicate by mailing copies to all users listed in the infected machine's email address book, masking the fact that they are viruses.

"Their second aim is to open a 'back door' on the infected machine that will allow someone other than the owner to take control remotely, which enables the unauthorised person to launch attacks on websites or get access to a virtual mailbox to distribute spam daily."

As such, Mr Poulton has devised a three-point plan to help businesses reduce the risk of infection.

l Consider any email that arrives with an unexpected attachment as a risk and delete accordingly. Temptations have included files described as pictures of Anna Kournikova or free screen savers.

l Ensure anti-virus programmes are installed on all systems and are regularly updated (at a minimum, systems should be updated on a weekly basis).

l Insist all business critical data is regularly backed up and that, where possible, these back-ups are stored off-site.

Mr Poulton said: "Most of the viruses use social engineering in an attempt to tempt the user to open the file that arrives via email.

"The key to minimising the risk of infection is to have a multi-layered defence which starts with educating users."