Billy's mommy had ties with IBM CEO John Opel, this is often overlooked but it's the key to MS success. Don't overestimate the philantropist , he always had a rich family.
http://www.nytimes.com/1994/06/11/obituaries/mary-gates-64-h...
She had been a regent at the University of Washington since 1975, the same year she became the first woman to serve as a director of First Interstate Bank and the first to serve as the president of the King County's United Way. She was later appointed to the board of the United Way of America; in 1983, she became the first woman to lead it. Right Time, Right Place
Her tenure on the national board's executive committee is believed to have helped Microsoft, based in Seattle, at a crucial time. In 1980, she discussed with John R. Opel, a fellow committee member who was the chairman of the International Business Machines Corporation, the business that I.B.M. was doing with Microsoft.
Mr. Opel, by some accounts, mentioned Mrs. Gates to other I.B.M. executives. A few weeks later, I.B.M. took a chance by hiring Microsoft, then a small software firm, to develop an operating system for its first personal computer.
And this...
http://prospectingprofessor.blogs.com/prospecting_professor/...
Why did IBM even go to Redmond, WA in the first place?
At the time, IBM president John Opel served on United Way's national board...as did Mrs. Gates. In 1980, when someone mentioned Microsoft to John Opel, he responded, "Oh, that's run by Bill Gates, Mary Gates' son." Behind the scenes, Mary Gates had spoken with John Opel about the new breed of small companies in the computer industry, which she felt were under-appreciated competitors of the larger firms with which IBM traditionally partnered.
That's not a mystery: it went to license Microsoft Basic, which was the most widely recognized Basic of its day. It dated back to the MITS Altair.
So IBM would have gone to Redmond regardless of Mary Gates.
However, it probably helped with the DOS deal, whereby Microsoft offered to save IBM's operating system bacon. Basic was a known quantity but MS-DOS didn't even exist.