After months of noisy prelude, the antitrust trial against the Microsoft Corp. opened in federal court Monday morning with a pointed personal attack on Bill Gates, the company's chairman. Microsoft, the world's leading maker of computer software, said demand for the Windows 98 operating system exceeded its projections, particularly in Japan and Europe. A lawyer for Microsoft Corp. on Thursday portrayed the company's competitor in the Internet software business, Netscape Communications Corp., as a willing and eager participant in deal-making that culminated in a June 1995 meeting at which the government and Netscape now say that Microsoft illegally offered to divide the market.