Dorothy Walker spends a hectic day making the rounds with a computer doctor
It happens to all of us - the moment when we face terminal illness. The computer screen fades to a flickering spot. The time has come to reach for the phone and call the computer doctor.

Steve Craigie mends, makes and maintains computers from a base in his office-workshop in Battersea, south London. His company, Systems Management Group, has a complex mix of customers: from private word-processors to embassies and blue-chip companies. He is a typical computer surgeon and I joined his small team for a day to find out just what does go wrong with computers and what he can do to save them.

09.05: A young man arrives, proudly brandishing a computer and two pieces of paper: "Look, I wrote these letters on my PC."

Nothing remarkable here - except that his machine's monitor wasn't working at the time.

For the past two days he has been flying blind, typing on to a blank screen, carefully counting every keystroke. Steve sighs the patient sigh of the man who has seen everything: "If he had only brought us the monitor when it first broke down, we could have loaned him one while we fixed it."

09.30: Finish recovering data for an author whose hard disk got corrupted. This is one of Steve's specialities. Even with a disk that seems to have expired, he will try to get the mechanical parts moving ("a good knock often makes the older ones go again"), and then use investigative software to piece together the muddled contents for a tearful owner.

10.45: Dash to an embassy in Belgravia to check on its computers. Steve dons a tie. No limousine, though - we get into a car littered with small pieces of communications cable, all neatly labelled "No Good". Computers are carried on the back seat, never in the boot - you need to cushion the effects of bumps.

We arrive at the embassy in the middle of a crisis about printing envelopes. After figuring out the word-processing software - which is all in the embassy's native tongue - Steve detects a bug.

13.15: Back in the cable car, heading for HQ, when there is a diversion. An accountant has called to report that her computer "simply isn't working". We head straight there. It looks fatal: at the ripe old age of five, the machine seems to have lost its memory. But on investigation, Steve finds the the culprit - a tiny and very old battery on the motherboard has run down and needs to be replaced.

14.45: Over lunch, Steve recalls a recent mystery: a PC that restarted itself twice every hour. He spent a day and a half dismembering and analysing it but could find nothing wrong.

When he returned it to the owner, he finally spotted the cause - an office ioniser, which had been very efficiently charging the computer with unwanted electricity.

15.15: Head for an Irish travel agency to install PCs for its electronic reservations system. Within minutes, the first machine is linked to Ireland and we begin what the computer industry calls the "live test": a customer has just walked in. He starts to haggle over prices.

After a few minutes, the exasperated bookings clerk turns to his PC and says triumphantly: "I'm sorry, but as you can see, these are the prices in the system." The customer pays up.

17.10: Return to base in Battersea to join John Racoveanu, who is tackling repairs that have been brought in. Steve is willing to deal with most computerised gizmos, although there wasn't much he could do for the laptop PC which had been in the path of a 12lb stone beaver that fell from a 6ft bookshelf.

As Steve said: "There is nothing you can do in cases like that but call the insurance man."

18.20: As he rummages through a mountain of obsolete components in the basement, he explains that PCs are becoming more robust. Mechanical wear and tear take their toll on moving parts (with hard disks still usually the first to fail) and badly fitting power leads cause surges in voltage which can wreak havoc. But the majority of problems on today's machines are caused by software. Rule number one: never panic and never switch off the machine in the middle of doing something.

19.10: A jogger arrives, wanting to know the price of modems. The conversation turns to e-mail, and by the time the man departs "to think about it", Steve and John have given the entire crash course on electronic communications. "Don't worry, he'll be back," says Steve. "He won't get that sort of service in the high street."

Does Steve surf the Internet? "I did one evening. I got to the Minnesota University department of biology and a pizza outfit in Cleveland, Ohio. I don't think it's for me."

20.30: An elegant couple dressed for dinner arrive to collect a multimedia machine Steve has upgraded. They pay by credit card, and Steve checks the office PC for the authorisation details. "No," says John. "Mirtha writes them in a notebook in pencil - she doesn't trust the computer."

I ask Steve what kind of PC he has at home. "A tatty old 486 - no multi- media or anything." And what would he buy to replace it? "I would advise anybody to buy a generic, Taiwanese IBM clone, built by someone you can trust. That way, you keep down your running costs, and you will always be able to upgrade it."

21.30: Steve explains that he wears many hats: doctor, forensic scientist, nanny, psychiatrist, linguist ... There is personal service in response to every phone call. And his energy is undiminished as midnight approaches. He sprints down to the basement again, this time to show me the active hi-fi speaker he designed in 1979. "I still maintain these for my customers."

He tells me that if anything goes wrong, particularly in small companies, the first person to get the blame is the man who maintains the computers. One manager even complained that his mouse was getting tired, which had to be Steve's fault. He is very patient with them, particularly when they are slow to pay bills: "You don't like to pull out the rug from under a small company - they depend on their technology. Once that's gone, they're finished."

23.49: Steve sends off a final fax to a customer, shuts the shop and heads off home to Kent.