'IBM Corporation today announced its smallest, lowest-priced computer system - the IBM Personal Computer,' ran the press release 25 years ago this week. 'Designed for business, school and home, the easy-to-use system sells for as little as $1,565. It offers many advanced features and, with optional software, may use hundreds of popular application programs.'

On 12 August 1981 no one could guess quite how profound an impact the announcement from International Business Machines would have on hundreds of millions of lives. Nor how wildly divergent would be the fortunes of three men who were there at the genesis of the IBM PC 5150 - a invention to rank in importance with the motor car, telephone and television.

One of those men was David Bradley, 57, a member of the original 12 engineers who worked on the secret project and who is still amazed by its profound consequences, from email and iPods to Google and MySpace. Speaking from his home in North Carolina last week, he said: 'Computers have improved the productivity of office workers and become a toy for the home. I don't want to assert that the PC invented the internet, but it was one of the preconditions.'

The man with perhaps most cause to toast the industry standard PC's 25th birthday on Saturday, even more than the engineers who built it, is Bill Gates. His software for the IBM PC, and nearly all the computers that followed it, made him the world's richest man. But for IBM, the story was arguably one of defeat snatched from the jaws of victory.

Bradley was also working on a similar machine when, in September 1980, he was recruited to the IBM team and sent to Boca Raton in Florida to come up with a PC that would rival the pioneering Apple II. A few months later the team had grown and got its leader - Don Estridge, a photographer's son from Florida who had worked for the army and Nasa. Racing against a 12-month deadline, the engineers scoured the country for components, and asked Intel, then a manufacturer of memory chips, to deliver the central processing unit, or 'brain'.

IBM also needed operating system software. The man in the right place at the right time was a young geek who had dropped out of Harvard. Bill Gates of Microsoft specialised in more modest computer languages but assured the IBM team that he could come up with an operating system for their new machines in just a few days. After Estridge's task force had left for their hotel, Gates went around the corner to a tiny company which had written a system for the Intel processor and bought it out for £26,000. He then customised the system for IBM and sold it to them for £42,000. Critically, Gates retained the right to license the system to other manufacturers who could, and would, clone the IBM design. A quarter of a century later, he has an estimated wealth of £26bn.

IBM's failure to secure exclusive rights to Gates's software is often regarded as a blunder comparable to that of the music executives who spurned The Beatles. But Bradley disagrees, saying that there was a higher purpose - he and his colleagues used 'open architecture', off-the-shelf parts which others could acquire, and so defined a standard that allowed others to build compatible machines capable of running the same software.

Experts generally regard this as the result of haste rather than altruism on IBM's part, but Bradley points out that in the spirit of openness it published technical manuals to explain how the PC worked. Unlike Apple, who stuck by its proprietary system and lost the lion's share of the market, the IBM PC was an invitation to rivals eager to imitate and improve upon it.

Bradley said: 'I believe the primary reason it was so successful is that it was an open system. There was a microprocessor from Intel and an operating system from Microsoft. We published everything we knew so that if you wanted to work on an application program you had all the information to do it and you could be reasonably confident IBM wouldn't change things later.

'The participation of the rest of the industry was important because IBM alone could not possibly have invented all the applications that people would want.'

The IBM PC 5150 weighed 25lbs, stood just under six inches high and had 64 kilobytes of memory and a five-and-a-quarter inch floppy disk drive. Initial sales forecasts expected 242,000 to be sold over five years, but the figure was exceeded in single month. It was a personal triumph for Estridge, the 'father of the PC', but he would not live to see its full legacy in the democratisation of computing.

On 2 August 1985 Estridge was on Delta Air Lines Flight 191 from Fort Lauderdale, Florida approaching Dallas-Fort Worth airport. It was caught in a freak wind and plummeted to the ground, bursting into flames. Of 152 passengers on board, 128 died, including 48-year-old Estridge, his wife and several IBM executives.

IBM was overtaken in the PC market by Compaq in 1994. IBM sold its PC division to Chinese giant Lenovo for £628m last year. 'I'm sad and disillusioned that IBM got out of the computer business since I was there at the very beginning,' added Bradley. 'But as an IBM stockholder I think it was an extremely sensible business decision.'

Bradley quit IBM in 2004 after 28 years and lives in comfortable retirement. He mused: 'I have no regrets about what happened. I was there when it was just a glimmer in everybody's eye and it's a privilege to still be here to talk about it. And no, I don't envy Bill Gates.'