Which software sent Bill Gates on his way to becoming one of the richest men the world?

DOS (Disk Operating System). He bought the program from Tim Paterson of Seattle Computer Products for $50,000. Then he released it for very low cost per unit to all the companies making IBM-AT colnes out of Japan. Since the operating system came with the machine, it became a defacto standard. Windows was a GUI written over the top of DOS for several versions, clear up to the Millennium edition.