Bill Gates was asked once "What is the secret to your success?" and his response was something to the effect "I was in the right place at the right time".

When IBM was developing the first PC in the early 1980s, it was between Microsoft and Digital Research as to who would supply the operating system. Digital Research actually had the contract, but when it came to crunch time, their O/S didn't work well. Luckily for Gates, he had previously decided to develop his O/S (MS-DOS) anyway, and was able to step into the breach created by Digital's failure.

At this point, IBM tried to buy MS-DOS outright from Gates, but Gates wouldn't sell, he would only license DOS to IBM. And the rest is history.

It's an interesting story, here is a good starting point to find out more.

MS-DOS - Wikipedia, the free encyclopedia