The co-founder of Microsoft alongside Bill Gates is Paul Allen. They established the company in 1975, focusing on developing software for personal computers. Allen played a pivotal role in Microsoft's early success, contributing to the development of software like MS-DOS and eventually the Windows operating system. Although he left Microsoft in the early 1980s, Allen's influence in the tech industry continued through various ventures, including investments in technology, healthcare, and space exploration. He remains a significant figure in the history of computing.