On July 27, 1981, Gates fully licensed the "quick and dirty operating system" (QDOS) from a company called Seattle Computer Systems, according to The Register. That OS would later become known as MS-DOS.

Not that long before this historical purchase, IBM came a'knockin' at Microsoft's door, in search of a 16-bit OS to run on its early PCs. Microsoft apparently thought it wiser to buy an existing solution than to build its own, so at first it non-exclusively licensed what was then called 86-DOS, or QDOS, from Seattle Computer Systems. Shortly thereafter, on July 27, Microsoft dropped another $50,000 for the exclusive rights.

In about a month's time, the software shipped on an IBM PC, according to Gizmodo. Later, MS-DOS became a legendary early OS. Microsoft and IBM started cozying up to each other in a relationship that would span decades and shape the tech world. Gates (and Cofounder Paul Allen and lots of other Microsofters) got very rich.

The rest is history.