An appeals court ruling to uphold a lower court decision that Microsoft illegally used its desktop operating system monopoly puts the software maker in danger of facing billions of dollars in damages from private antitrust lawsuits, analysts and legal experts said Friday.

While a conclusion to the government's antitrust case could still be years away, competitors that can argue they were harmed by Microsoft's monopolistic behavior gained from Thursday's decision. Already Microsoft faces more than 100 private antitrust lawsuits from plaintiffs claiming that the desktop operating system monopoly led users to pay inflated prices for Microsoft's software. When the government's case against Microsoft is fully resolved, experts say the company could face scores more.

"I think any company that can make a case that it has been damaged by the exclusionary acts affirmed in the Court of Appeals opinion would be able to use the facts from this case once the case becomes final," said Dana Hayter, an attorney at the law firm Fenwick & West and former member of the U.S. Department of Justice (DOJ) in San Francisco.

Several additional lawsuits could come from some of Microsoft's biggest rivals, including AOL Time Warner and digital media company RealNetworks.

Microsoft, of course, denied that competitors have gained a legal advantage over the company. "There's nothing in the court of appeals decision or the original decision that supports the contention that Microsoft overcharged consumers," said Jim Cullinan, a Microsoft spokesman. "We believe that there is no validity to these suggestions. And we believe so far that we have been successful in many of the private class-action lawsuits."

A federal appeals court delivered a ruling Thursday in the landmark antitrust case between Microsoft and the U.S. government, sending parts of the case back to a lower court for review by a new judge, while upholding the lower court verdict that Microsoft acted illegally to maintain its monopoly. The appeals court also overturned an order from U.S. District Judge Thomas Penfield Jackson that Microsoft be split into two separate companies as a remedy for its illegal anticompetitive behavior. Jackson was disqualified from hearing the case again, so the new judge will also reconsider the remedy.

That decision -- and any others that follow as the case proceeds -- give private plaintiffs a key weapon in state and federal courts. In legal speak that's called "Res Judicata." Any plaintiff that has been damaged by Microsoft's monopolistic behavior can introduce Judge Jackson's "findings of fact," which still stand without having to prove them again. If a plaintiff can prove evidence of liability, "then all they have to prove is damages," Hayter said.

Since Jackson's order last year to break Microsoft in two, an estimated 140 private and civil lawsuits have been filed in various state and federal courts. By some estimates, Microsoft has overcharged customers US$10 per copy of its operating system, according to Robert Hall, an economics professor at Stanford University and senior fellow at the Hoover Institute, who is involved in the government's case against Microsoft.

In January, A U.S. district judge in Maryland dismissed 20 of the 62 class-action lawsuits and parts of 18 others, noting in his ruling that Microsoft was only liable to plaintiffs who were direct purchasers of the Windows operating system. In addition to the huge federal case in Maryland, a few other cases have been thrown out at the state level.

But 20 states allow individual purchasers to file claims, and the appeals court opinion has paved the way for more companies to take advantage of that right.

The appeals court ruling most benefits plaintiffs that compete in the same market as Microsoft's operating system because that is the one part of Jackson's original verdict that was upheld as monopolistic, Hayter said. Companies working on competing software development platforms, such as Sun Microsystems with Java, could also make a case, though that would be difficult because Java has been relatively successful in the market.

It will be more difficult for prospective plaintiffs to make a case when it comes to the Web browser market, Hayter said, because the appeals court opinion argued that market was not clearly defined. "It's hard to know without knowing what market, how much this case helps them," he said. "But it sure doesn't hurt any of them."

One of its biggest threats comes from AOL Time Warner, the parent company of Netscape Communications Corp., which was one of the original victims of Microsoft's anticompetitive practices. Netscape's Navigator Web browser is at the center of the government's case against Microsoft.

"We have seen numbers quoted that indicate Microsoft could owe damages to the tune of $3 billion to Netscape/AOL," Laura Lederman, a financial analyst with William Blair & Co. in Chicago., wrote in a research note Friday.

Some observers say legal action from AOL is unlikely, noting that the media company is in a better position to use the legal ruling to forge some sort of deal with Microsoft.

"Of course AOL has a grand opportunity to file suit and claim damages," said Richard McKenzie, a professor of economics at the University of California, Irvine Graduate School of Management. He doesn't expect that, however. "It would come down to a cost-benefit analysis from them."

AOL positioned itself last month so it could potentially bring litigation against Microsoft, though officials have not made any signs that the company will seek legal action. Partly over the issue of rights to sue, AOL and Microsoft ended discussions to continue a relationship where AOL would support Microsoft's Internet Explorer browser in exchange for maintaining a marketing relationship with the release of Microsoft's Windows XP operating system.

Executives from both companies said they would continue to work together, but the dealings ended in part because AOL officials said they would not give up their rights to sue Microsoft, a concession the software maker had required.

A number of other companies in various markets could also come forward with their own private cases.

"If Microsoft introduces Windows XP as planned with the messaging applications and the media player bundled with it, it seems to me that companies like RealNetworks would have a basis for filing a suit," McKenzie said.

Microsoft has said it will bundle its Windows Media Player 8, which competes against RealNetworks' RealPlayer, in the new operating system. Applications for text, voice and video will also be included free with messaging software. Microsoft competes against AOL and Yahoo Inc. in the instant messaging market.

PC hardware vendors could also make a case to sue, according to Lederman. Companies such as Dell and Compaq could argue that Microsoft overcharged them for Windows, in effect owing them a rebate for all of the software packages they sold to consumers. Others in the industry such as Intel, Sun and Apple could also potentially make a case for lawsuits if they can prove that it was Microsoft's monopoly power that put them at a disadvantage and not internal problems or external issues that made them weak in their respective markets, Lederman wrote.

The 19 state attorneys general who are plaintiffs in the government's case have also hinted that they will continue their legal assault on Microsoft based on its plans with Windows XP and its plans to build an operating system for the Internet with its .Net initiative.

Within the next six years, analysts predict the legal bills could pile up for Microsoft, which has about $30 billion in its coffers to fend off legal action. For one, plaintiffs successful in private antitrust litigation get what's called "trouble damages," in addition to attorney fees, Hayter said. That means they are awarded three times the damages they ask for. In many of the private antitrust suits, damages could reach several billion dollars.

"The court clearly said that Microsoft is a monopoly that has abused its market position," said McKenzie, who wrote a book on the legal ramifications of the landmark antitrust case. "As a consequence one can presume some consumer harm from that.

"That greatly eases the burden for these private suits," he said.