Bill Gates at 50

What do you get the man who has everything? Spare a thought for the friends and family of Bill Gates: buying presents for the world's richest man is the kind of puzzle that few gift-givers would relish.

Tonight Gates will blow out the candles for a personal landmark: his 50th birthday. In fact, it's been a year of big anniversaries for Gates - last month Microsoft, the monolithic software company which made him a billionaire, turned 30. It's hard to believe that the man - and his machines - are so young: for someone who's only just hitting his half century, Gates has packed in a remarkable amount.

It took Coca-Cola about 30 years to reach the stock market, and at the same age McDonald's was just opening its first restaurants outside the US. Global domination? He's been there, done that and bought the company that made the T-shirts.

As befits someone who has achieved so much, Gates has already seen several distinct phases in his life. He started out as a pre-pubescent programmer, then sold his first piece of software for $4,000 at the age of 17.

Gates then made his way to Harvard, before dropping out. What followed were his halcyon days as the king of the nerds: a successful entrepreneur who built an incredibly successful company and helping define the computing boom that took hold in the 1980s.

Even if his fashion sense was worthy of Napoleon Dynamite, his business acumen made a big impact, and in 1984 he first hit the cover of Time magazine - one of those milestone moments that mark out the rich and the powerful.

A period of empire-building was followed by massive success for Microsoft. But it came at a cost: a series of anti-trust lawsuits and accusations of monopolistic behaviour hit the firm hard - Gates had created the biggest company on the planet, but turned into pantomime villain in the process.

It's not a hard step for the imagination - after all, his personal power and extraordinary wealth puts him way ahead of any media mogul in history. And his obsessions - computer code, brain function and, reputedly, the size of his IQ - are exactly the kind of fixations that make him seem more like machine than man.

Now, though, with anti-trust lawsuits out of the way, Gates almost has the air of a dilettante. He stepped back from day-to-day running of the company in 2000. He remains the chairman, but more of his time is spent with his wife and children in their massive hi-tech mansion outside Seattle that's worth in excess of $100m.

Now Gates is entering the third stage of his career - a life as philanthropist, campaigner and, increasingly, political heavyweight.

He's a Renaissance man, literally, with a library that houses a fortune in valuable manuscripts, including original work by Leonardo da Vinci. And when he fancies doing something more interesting than pottering around the house, Bill goes and signs a cheque or two.

Through the foundation he set up with his wife, Melinda, Gates has pumped billions of dollars into charity organisations, particularly ones that aim to halt the spread of diseases in the developing world.

In January he forked out $750m for vaccinations in Africa, and the foundation has a total endowment of around $28bn.

So what's next for William Henry Gates III? After all, 50 is nothing. He's rich, powerful and has more cash in his back pocket than a small country.

But while he keeps fairly quiet, it doesn't look as if he's going to do a Howard Hughes and disappear completely. No, it's much more likely that Gates - like any good geek - is going to just keep on going. And going. And going.

After all, it what he's programmed to do.