Going public is one of capitalism’s major sacraments, conferring instant superwealth on a few talented and lucky entrepreneurs. Of the more than 1,500 companies that have undergone this rite of passage in the past five years, few have enjoyed a more fren- zied welcome from investors than Microsoft, the Seattle-based maker of software for personal computers. Its shares, offered at $21 on March 13, zoomed to $35.50 on the over-the-counter market before settling back to a recent $31.25. Microsoft and its shareholders raised $61 million. The biggest winner was William H. Gates III, the company’s co-founder and chairman. He got only $1.6 million for the shares he sold, but going public put a market value of $350 million on the 45% stake he retains. A software prodigy who helped start Microsoft while still in his teens, Gates, at 30, is probably one of the 100 richest Americans.