The original complaint for the US case remains available online from the DoJ: http://www.justice.gov/atr/cases/f1700/1763.htm

There are four bases for the complaint, two under Sec. 1 of the Sherman Anti-Trust Act and two under Sec. 2 of the Act.

1. Unlawful Exclusive Dealing and Other
Exclusionary Agreements in Violation of Section 1 of the Sherman Act (deals with ISPs, Internet Content Providers, and others that traded favorable promotional placement in the then-monopoly Windows OS for commitments not to license, promote, or distribute non-Microsoft products, particularly browsers.)

2. Unlawful Tying in Violation of Section 1 of the Sherman Act (distributing the browser, a separate product based on market demand, etc., with its monopoly Windows OS system.)

3. Monopolization of the PC Operating
Systems Market in Violation of Section 2 of the Sherman Act (willfully maintaining monopoly power over that market by "by anticompetitive and unreasonably exclusionary conduct.")

4. Attempted Monopolization of the Internet
Browser Market in Violation of Section 2 of the Sherman Act (targetting "software products that have the potential to compete with or facilitate the development of products to compete with PC operating systems and thereby to erode Microsoft's Windows operating system monopoly" by way of "including tying and unreasonably exclusionary agreements, in order to obtain a monopoly in the Internet browser market".)

Note that a key issue in all of these is Microsoft's then-existing monopoly power in the PC OS market and how it was defending or leveraging that in different markets -- without that monopoly power, much of what they were doing would not have even arguably been an antitrust issue.

Browser+OS wasn't the issue, it was one of the mechanisms -- unreasonable and anticompetitive action to maintain an existing monopoly, and to leverage that monopoly into a monopoly in another market, was the issue.

> However it still doesn't explain why Internet Explorer was the target when there is so much other software that was bundled with Windows, which too had for profit competitors?

Because the browser was the area where the government felt they had the best case for two key features: (1) that the tying, etc., was intended to leverage the existing monopoly to monopolize another market, and, (2) that the tying, etc., was intended to neutralize a threat to Microsoft's existing monopoly.

Bundling two products together isn't generally illegal. Doing so as a means to leverage a monopoly in one market to monopolize another, or to defuse a threat to an existing monopoly (and, a fortiori, to serve both purposes simultaneously) is a different story.

> Those people who exclusively used Internet Explorer as "the Internet" would too have used the bundled mail client (I can't remember what it was called back in the day) exclusively too and seen that as "the email"?

The bundled email client wasn't an issue because it didn't exist yet -- Outlook Express wasn't bundled with Windows until Windows 98, which was still pending at the time the lawsuit was drafted (the complaint notes at para. 57: "Beginning in or around June 1998, Microsoft will introduce to the market the latest version of its operating system for Intel-based PCs, Windows 98.")