Scott McNealy, the chief executive of Sun Microsystems, is probably the world's leading Microsoft critic, a man who never met a Microsoft product he didn't dislike. Even so, it's worth paying attention to an argument McNealy made in an interview last week, because it could change the terms of the Microsoft antitrust debate.

McNealy argues that the Justice Department case is missing the most important and potentially dangerous aspect of Microsoft's behavior: the company's aggressive investment in other technology and telecommunications companies.

Ever since Justice filed its suit a year ago, he contends, Microsoft has been racing to invest the profits of its Windows monopoly in companies that will control the next wave of technology. If the court doesn't stop Microsoft, he warns, it will use the illegal fruits of one monopoly to buy its way into the next.

Never shy about offering advice, the Sun CEO has a recommendation for what Judge Thomas Penfield Jackson should do about the problem. He argues that if Judge Jackson finds against Microsoft, he should order the software giant to sell its investments in other technology and telecommunications companies. That, he says, would be "the most important remedy" in the case, dealing with what he calls "the biggest issue, by far."

McNealy's comments are important, because they're the first major public alarm bell about the potential danger of Microsoft's investments. The Justice Department's antitrust division has already begun quietly analyzing the investment issue, which raises some novel antitrust questions. And the issue may surface this week, when both McNealy and Microsoft chief executive Bill Gates are scheduled to testify before Congress.

"What they can't do with R&D, they're trying to do with M&A [mergers and acquisitions]," grumbles McNealy. "So they're taking $4 billion a quarter in cash and investing -- before the trial is over -- and going beserk-crazy buying customers for their technology."

An example of what worries McNealy was Microsoft's $5 billion investment last month in AT&T, which has become the new cable giant after its recent purchases of TCI and MediaOne. "Ma Cable" hopes to provide Internet access and telephone service through your cable wire -- in addition to cable TV.

And guess what software AT&T has agreed to use in many of the "set-top boxes" that will drive the system? Microsoft's Windows CE, of course! That means that if AT&T succeeds in its goal of dominating Internet service to the home, Microsoft will have locked up part of the market for what may be the most important post-PC operating system.

This cozy relationship concerns McNealy -- and prompts one of his famous outbursts. "I don't know anyone who thinks CE is good stuff," he complains. "CE is a trimmed-down Windows hairball that doesn't even run Windows applications. You get the worst of both worlds!"

McNealy sees a similar pattern in other recent Microsoft investments. The software giant, understanding that its old desktop PC monopoly is under threat, is using its war chest to buy a favored position in the new array of computing devices that will supplement the PC. For example, Microsoft's $600 million investment last month in Nextel could give Microsoft the inside track in supplying software for that company's wireless Internet services.

Here's a list of additional cable and telephone companies Microsoft has invested in since the trial began a year ago, as compiled by McNealy's staff: TeleWest, which provides cable and telephone services; Qwest, a telephone company; SkyTel, a paging company. Earlier investments included UUNet (now part of MCIWorldCom), a telecommunications company; Mobile Telecom Technologies, which provides cellular phone services; and Comcast, a cable company.

Outright acquisitions by Microsoft include its 1997 purchase of WebTV, which provides Internet access via television.

Forcing Microsoft to sell these investments would be unusual. But antitrust experts say the terms of the 1984 AT&T breakup offer one precedent for forbidding a monopolist from investing in related technologies.

What infuriates McNealy is that Microsoft is using its vast hoard of cash to buy into a post-PC world that Sun, more than any other company, helped to create. It was Sun's new computing language, Java, and its connectivity software, known as JINI, that helped make the new paradigm of pervasive computing and interconnectivity possible.

Microsoft initially resisted many of these innovations, McNealy claims, but now "they understand that it's unstoppable. . . . They can't dominate this next generation unless they leverage their current monopoly, which is exactly what they are doing."

McNealy still thinks the market is going Sun's way. Currently, he says, more than 85 percent of the devices that access the Internet are Windows-equipped PCs. But by 2002, he predicts, that number should drop to less than 50 percent. "At that point, Microsoft starts to lose power," he explains. "That's why they are on this mad dash, before the antitrust case is closed, to lock up the new world through mergers and acquisitions."

Poor Judge Jackson. He had a complicated case already. But in these final weeks of the antitrust "trial of the century," he may want to ponder the issues McNealy is raising.