She had just turned 40 and had a long list of things that she wanted to do with her husband and

She had just turned 40 and had a long list of things that she wanted to do with her husband and two teenage children, including a trip to Africa. After eight years as the top female executive at Microsoft, Patty Stonesifer was ready to retire a multimillionaire. Her division, now called Interactive Media, had mushroomed into the largest multimedia company in the US. She had just turned 40 and had a long list of things that she wanted to do with her husband and two teenage children, including a trip to Africa. That was in 1997, but today Stonesifer is still hard at work for Bill Gates. Even she sounds a little amazed describing how her boss lured her back to a job which pays no salary and for which she had no experience.

Her leaving party was in full swing when Gates approached her with an offer that she could not refuse: instead of making money for Microsoft, he wanted her to help give it away.As chairwoman of the Bill and Melinda Gates Foundation, Stonesifer has the Midas touch. The foundation, with over $21bn in assets, of which it spends $1bn each year, is the most richly endowed philanthropic organisation on earth. It is worth more than the foundations bearing the names Carnegie, Rockefeller and Ford ever gave combined– even when their donations are converted into today's dollars.The challenge for Stonesifer is deciding where the money will work most effectively – whether it is buying a computer for a Rotary Club in South Dakota or helping to find a vaccine for Aids Stonesifer was brought in to help Bill Gates Snr. Now her co-chair, he was working out of the basement of his house when his son and daughter-in-law made it their mission to spend their money before they die. Since then, the team has grown to more than 150 employees, two-thirds of whom are working to put computers in every library in America."It was something I was so well suited to do after spending all those years in technology," Stonesifer says "I operate the business of a philanthropic foundation.

Bill and Melinda are at the strategic helm, but it is my job to help shape strategy and execute it." If it was the opportunity to travel that Stonesifer was looking forward to when she quit her job at Microsoft after 20 years in the hi-tech industry, she has certainly had her wish realised. Field trips to India and Africa are a key part of her latest role as more than half the funds from the foundation go towards combatting disease in the developing world. "Our biggest area of focus is children's vaccines – increasing access to ones that are already developed and developing new ones," she says. "Bill likes to say vaccines are almost magic because they prevent such bad health for such an inexpensive investment."Coming in at entry-level, Stonesifer, an Indiana University graduate, has had plenty of catching up to do "I'm learning every day," she says. "There I was, a college-educated woman who read magazines and newspapers; I considered myself a political and social activist. But I knew almost nothing about what was happening in terms of the incredibly rapid increase in HIV transmission through Africa."In January, the foundation committed $100m to the search for an Aids vaccine.

In April, Gates held a news conference to respond to a group of 120 academics from Harvard University who are calling for wealthy nations to buy drugs for Africa and other developing nations. Two weeks ago Stonesifer was in Brussels to announce that the foundation was donating a further $100m to a United Nations health project to fight the spread of the disease."There's a sense of urgency when you're participating in a moment in a technology revolution," she says "We feel that same sense of urgency now Science is accelerating at an incredible rate. This is the time to act." Every day she has to make trade-offs weighing up where money would most effectively be placed. Around 3,000 requests arrive at the foundation's headquarters in Seattle every month. Of those, only 100 get a "yes"."It's important for me to feel agitated about it or I wouldn't take it as seriously as I need to with the opportunity to create change that we have," she says. "There is a tension, realising that every choice you make is a choice not to do other things. We use that tension to hone our thoughts on what we should concentrate on."Stonesifer – tall and slim with red hair – says her latest job has been life-changing.