These so-called "shell" screens were offered by some PC makers until a few years ago, when Microsoft tightened its contract restrictions, industry executives say.

One step Microsoft had taken in the last couple of years, the Justice Department contended, was to tighten its contract restrictions on PC makers.

The Government argues that Microsoft's decisions to bundle its browser with Windows and to place contract restrictions on personal computer makers add up to anticompetitive behavior.

Similarly, Microsoft relaxed some contract restrictions on Internet service suppliers early this year.

Beyond the luxury tax and contract restrictions, another obstacle to widespread movement and trades, two team officials said, is the activity of a few teams.

Pixel, its managers say, was born of a sense of frustration with Microsoft's contract restrictions.

And Microsoft argues that many of its contract restrictions have since been eased.

All three agencies also sell to budget travelers who are neither young nor students nor teachers, although the prices are higher because of contract restrictions.

The suits will contend that Microsoft used predatory pricing and contract restrictions to extend its monopoly.

"Those contract restrictions were absolutely legal, but we've decided to drop them because they attracted attention in Washington and elsewhere," Mr. Herbold said.