But, while the spotlight will be on Microsoft, the Redmond, Wash.-based company isn’t the one with the most to lose from the closely-watched court decision.

The case is seen as a key test of Europe’s role as one of the business world’s most powerful watchdogs.

If European regulators succeed in curtailing Microsoft, some legal experts say it will embolden their aggressive stance. On the flip side, a Microsoft win that reins in the watchdogs could make it easier for large businesses to use tough tactics to keep upstarts at bay.

A number of Silicon Valley companies have run into Europe’s crosshairs – Apple (AAPL) for its dominance in digital music, Google (GOOG) for its stranglehold on the Internet advertising business and Intel (INTC) for its lead position in the computer chip sector.

“This is easily the most important antitrust case of the last 10 years,” said Randal C. Picker, a professor at the University of Chicago who teaches antitrust law. “As a practical matter, Google and other companies will watch this decision very closely.”

The roots of Microsoft’s troubles in Europe date to 1998, when regulators began looking into the company’s monopoly in operating system software and its practice of bundling new products into Windows, which runs more than 90 percent of the world’s computers.

In 2004 – three years after Microsoft settled an antitrust case with U.S. regulators over the same bundling tactics – the European Commission socked the company with a record fine of 497 million euros (about $690 million). The commission found that that Microsoft unfairly used Windows to lock out competition in the market for media player software and workgroup servers.

Last year the commission ordered Microsoft to pay another 280.5 million euros (about $390 million) for not complying with its earlier ruling.

Those fines and the E.U.’s underlying findings are the subject of Monday’s long-awaited court decision. The ruling from the European Court of First Instance, the Luxembourg-based court that reviews commission cases against businesses, is expected at 3:30 a.m. eastern time.

While few expect Microsoft to get off scot-free, it could take weeks for legal experts to parse through what is expected to be a long and nuanced decision.

This much is for certain: A lot has changed since the E.U. first brought charges against Microsoft. For one thing, the company is no longer the fearsome giant it once was. It has already, for example, bowed to some of the E.U.’s demands by, among other things, selling a version of Windows without a media player and licensing its server technology to rivals such as Sun Microsystems (JAVA) and Novell (NOVL).

“When we started the U.S. case and the E.U. case, Microsoft really was the 800-pound gorilla, and it’s not that now,” said Picker, the University of Chicago law professor.

All eyes instead are on Silicon Valley’s rising stars and what Monday’s ruling could mean for them.

First up is Google, whose seemingly unstoppable march into the online advertising market worries European regulators. Already they’re scrutinizing Google’s planned buyout of advertising giant DoubleClick – a $3.1 billion deal that rivals and publishers fear will give the company too much power to set online advertising rates.

“The way in which Google is expanding on the web is similar to the way Microsoft expanded on the desktop,” said Picker.

Meanwhile, the European Commission is questioning Apple’s pricing of songs downloaded at its iTunes Music Store, which CEO Steve Jobs boasted was the No. 1 online music store in each of the 22 countries in which it operates. Among other things, regulators are looking into Apple’s practice of charging more for downloads in some countries than in others.

As for Intel, European regulators accuse the computer chip giant of slashing prices and doling out improper payments to unfairly shut out archrival Advanced Micro Devices (AMD). E.U. officials contend that Intel, which controls 75 percent of the chip market, cut prices so deeply that AMD and others sacrificed profits to compete.

“When a company has monopoly power, how much control do they have over their products? That’s the central question [in the Microsoft] case,” said Matt Rosoff, analyst for Directions on Microsoft, an independent firm that covers that company.

Worst-case for Apple et al., the appeals court rules against Microsoft and issues detailed guidelines for regulators to follow as they consider bringing anti-competitive charges against them.

If the court says dominant companies must make concessions to competitors, that could influence whether Google can use its search engine as leverage to enter the phone business, or whether Apple can keep its iTunes service closed to devices that compete with the iPod and iPhone.

In the end, there’s a chance that this is not the end of the road for Microsoft’s antitrust woes in Europe. Even if both sides are satisfied with the appeals court decision, Microsoft could run afoul of regulators again.

These days, however, Google and Apple are in the hot seat.