By Malathi Nayak SAN FRANCISCO (Reuters) – Satya Nadella’s elevation to CEO and co-founder Bill Gates’ decision to relinquish his chairmanship at Microsoft Corp marks a changing of the guard at a 39-year-old company that fueled the PC revolution but is now struggling to establish its long-term identity. Here are some highlights in the four-decade journey of the world’s largest software maker: 1975 – A 19-year-old Gates drops out of Harvard University and goes on to found Microsoft with childhood friend Paul Allen. …