What software was sent bill gates on his way to becoming one of the richest men in the world?

Several pieces of software helped to position where Bill Gates is now. Although not terribly relevant today in it's own right, I think BASIC probably helped him in the right direction. IBM asked Microsoft to port BASIC to their IBM PC. Since Digital Research did not license CP/M to IBM, and Microsoft had a working relationship with them, they were asked to write the system's software. Microsft bought the rights to QDOS, made some changes, and it became known as PC-DOS. IBM did not stop Microsoft from selling the software to other companies, so they did, renamed as "MS-DOS." And so Microsoft began gaining money, because of the growth of the IBM PC-clone industry.