Antitrust laws exist mainly to “prohibit agreements or practices that restrict free trade and competition between business entities.”
So, among other things, they protect you from Company X trying to force you to buy a second product after you’ve already bought a first one. This is the main crux of what got Microsoft in trouble with Internet Explorer.
These laws, specifically the Sherman Act, are likely what’s keeping you and me from having to buy our milk from Standard Oil, which is great, but with software it can be tricky. Microsoft stepped over the line a bit for sure, but it’s not always so blatant. What constitutes a “product”? Where does one piece of software end and the other begin? What should be part of the operating system and what shouldn’t? For the most part I think software companies are doing it right. Applications are focused around particular tasks: listening to music, getting your email, browsing the web, editing photos, etc., and the operating system or “the web” is there to facilitate that. It’s the links that make these individual applications really shine and that’s where this gets sticky.