I WAS drinking a cup of tea when I came under attack. I had just booted up the computer in our office last Thursday and was idly wondering which of the day's tasks to tackle first when it became clear that I was the target of a sustained assault.

The thing about your average attacker is you can see them; they may be wearing a balaclava, perhaps wielding a baseball bat, and probably uttering unpleasant grunting sounds.

However, the computer virus attacker moves silently; you never see him (they are usually men, although Internet security experts say some of the best creators are women) but from their dark little back rooms they are capable of launching an onslaught of epic proportions.

Like most computer users, I was aware that something was going to happen because security companies had been e-mailing warnings for days.

Sometimes the industry is accused of crying wolf but often it is right and the first warnings a week or so ago referred to an electronic "worm" programmed to attack computers with inadequate software protection.

By Monday last week, computers with some Microsoft software packages had already been hit and industry experts were describing it as one of the most serious incidents they had ever dealt with.

W32.Blaster - also known as W32.Lovsan - is spread from computer to computer via e-mail, seeking out those without a security 'firewall', the software which prevents nasties getting into computers.Thanks to the computer industry's vigilance, the worst excesses of Blaster were avoided.

But our business was not attacked by Blaster. No, just as everyone was relaxing, came the type of onslaught the likes of which I had never seen before, from a virus known as Sobig F.

By Friday morning, the industry was claiming this virus to be the worst ever. Welcome to the week's second Doomsday.

Like Blaster, Sobig F infects a machine then e-mails the virus to everyone in the online address book. It is the most common ploy among those who inhabit the shadowy world of the virus writer. After all, you are less likely to suspect an e-mail from someone you know. Indeed, during last week's Sobig F attack we received a e-mail from a hotel which we used on a business trip, warning that they had been infected and had inadvertently sent out virus-laden messages to clients. Not good for their image.

Between Wednesday night and lunchtime on Thursday last week , our security software identified wave after wave of attempts to infect us with Sobig F. Forty in all. Normally we only get one or two a week. Sobig F, which also caused problems two months ago, is a cunning little number which fakes an e-mail address to hide its origins and regularly changes its form and the messages it creates to make it harder to spot. The virus-laden e-mails aimed at us bore messages such as 'approved', and 'wicked screensaver', and others have said 'thank you' and 'that movie.' The idea, as with all viruses, is to trick the unwary into opening the attachment.

Remember the I Love You virus of a year or two ago? Its creator played on vanity, bargaining that there were enough people daft enough to want to find out about a hidden admirer. Open the attachment and that was it.

Our business survived Sobig F because we have two security systems in place: the system used by our Internet provider and additional software which we bought from one of the big security companies. As I write - and I am touching every bit of wood I can find - our security has held against Sobig F.

But I already know exactly what a virus can do because last year, one got through and threatened our business.

I had just updated our software. Trouble was, I made a mistake and accidentally lowered our defences for a short time without realising it. There is nothing a virus likes more than a fool and the attack came in the form of sircam, a notorious virus carried on an attachment.

The moment I realised we had been infected, I used a seek-and-destroy software programme to get rid of the virus but many of our operating systems were affected. Thankfully, my brother, who works in the computer industry, was able to rebuild my programmes without loss of data.

It could have gone the other way and four years worth of work could have been lost. Yes, it was backed up on discs but transferring it back to the machine can take a long time. And there was no guarantee that the machine itself would work properly again, so serious was the damage. We were fortunate, within three or four days of infection we were back working, desperately chasing deadlines but back working.

It was a salutary lesson and now I am a religious adherent to security, updating my software every few days just as I lock the back door of my house at night. It is a good analogy to use.

Last week, faced with the Sobig F onslaught, the software did its job but plenty of computer users, domestic and business, will have discovered the full price of that false economy when they decided not to buy in protection.

And a final chilling thought: most virus writers are young people who do it to show how clever they are and have only the loosest of liaisons with each other.

However, many Internet security experts fear something more sinister, that the next big terrorist attack will not be a hijacked airliner slamming into a tower block, rather a virus which wheedles its way into nuclear power stations, military installations or air traffic control systems.

Fact or fiction? The day we find out will be a black one indeed.