EACH SUMMER BILL GATES TREATS friends and associates to a party at the family home on Washington State's Puget Sound. But these affairs aren't all chips and dips; the host also arranges contests ranging from sand-castle building (for which he once trucked in six tons of sand) to this July's paint-gun duels (mercifully, with wash-off paints). It is this competitive spirit that has vaulted Gates, a still-tender 35, atop the computer software industry. A multimillionaire since his company, Microsoft, went public in 1986, he is today worth some $4.4 billion—good for No. 3 on Forbes's 1991 list of richest Americans.

"When you have success," notes Gates, "people seek to explain it." In his case, the explanation is MS-DOS, the Microsoft Disk Operating System that IBM selected to enable the launch of its first personal computers. By itself, a PC is a rudderless piece of hardware; it needs software designed for specific tasks—and an operating system that permits hardware to interpret software. Today some 70 million IBM PCs and compatibles, which make up more than 80 percent of the world market, operate on MS-DOS. The licensing royalties are funding development of the company's other programs in progress, which include word processing, spreadsheets, games and the best-selling new Windows. Microsoft is now the third-largest company in the industry (behind IBM and Hewlett-Packard), and Gates, its chairman, retains a third of the company's shares.

Alone among the young computer hackers who in the 1970s booted up the PC revolution, Gates has an aggressive approach to business matters. Says industry newsletter publisher Richard Shaffer: "Bill believes that if you hit a man, make sure he doesn't hit back." Others are less tactful. Snapped software rival Philippe Kahn of Borland: "No one likes to do business with Microsoft." Gates seems inured to such attacks. "Name a year," he said recently, "I'll name the people out to get us." And despite his critics, Gates insists that "I like how business and technology come together—it's on that boundary where neat stuff happens."

William H. Gates III has been at that intersection since junior high. The only son of William Jr., a Seattle lawyer, and Mary, a University of Washington regent, Bill was an underachieving math whiz who resisted entering the exclusive Lakeside School. "I seriously considered flunking the exam, but the ego challenge overcame me," he admits. Yet there he caught the computer bug and spent his final semester working as a mainframe programmer.

In 1974, while attending Harvard, Gates heard from Lakeside chum Paul Allen, who had spotted an ad for the world's first home computer—the assemble-it-yourself Altair 8800. The two worked nonstop for six weeks to devise an operating system for it. When their program worked, Gates dropped out to found Microsoft with Allen. Five years later, Gates sold IBM on the merits of MS-DOS.

Although bachelor Gates is building a 37,000-square-foot Xanadu, he maintains that wealth "loses all power to motivate once you have enough to be comfortable. I had enough 15 years ago, so it has zero impact right now." Instead, he still logs 75-hour workweeks at Microsoft's 21-building complex in Redmond, Wash., working on ways for computers to read handwriting and developing multimedia machines that will tap CDs for high-resolution images and sound, including music.

"This is a very real-time environment—this is fun stuff," he observes. "Computers require immense imagination. We're designing cool things here, and when it works, I still get very excited. In 1975 I put down as my goal that I wanted a computer on every desk and in every home. I'm 10 years away from achieving that."

But what if achieving it results in a nation of computer headcases? "You can wony that people will sit in their houses all the time addicted to these things," Gates concedes. "Any change [of that magnitude] comes with its side effects that aren't good. But I tend to think access to information is an empowering thing. And while we can ask these questions, it's also part of progress—it's going to happen."

—T.C.; NICK GALLO in Seattle

Get up-to-the-minute celebrity news and photos on your cellphone, iPhone or Blackberry at www.people.com!