Global shipments of personal computers grew 21.7 percent in 1996 from the previous year to 72.2 million units, paced by purchases in the U.S. by corporate buyers, a major Japanese think tank reported Jan. 30.Nomura Research Institute, affiliated with Nomura Securities Co., predicts that global PC shipments this year will grow 22.9 percent to 88.5 million units. This rise will be aided by the the penetration of Microsoft Corp.’s new network operating system, Windows NT4.0, they said.Shipments of portable PCs rose 29.5 percent, while those of desktop models increased 20.1 percent. The report said this reflects the narrowing cost-performance gap between the two types.In terms of companies, Compaq Corp. kept its No. 1 ranking, controlling 11 percent of the market. IBM Corp. had 9 percent, while Apple Computer Inc.’s share declined to 5 percent, as more corporate customers traded their Macintosh systems in for Windows machines. Japanese manufacturers, such as NEC Corp., Toshiba Corp. and Fujitsu Ltd., increased their share in the global marketplace, backed by rapid growth in the Japanese PC market.