Bill Gates, Philanthropist

Bill Gates is one of the few people who can claim to have changed the way the world works. Back when he was a teenager he had a vision of cheap computers on every desk in every home. Microsoft made him the richest man in the world. But his main work now is with his charity, the Bill and Melinda Gates Foundation, which focuses on tackling malaria, AIDS, poverty and disease. Drawing on his past achievements, he remains a fervent advocate of the power of technology to improve the world. And still a computer geek at heart, he tells Tom Hagler why he prefers Twitter to Facebook.