Microsoft released an Operating System where the main portal of entry to the internet was also their own product. Microsoft engaged in anti-competitive behavior to dissuade users from having alternative browsers installed -- which is similar to a shopping mall starting its own jewelry store to compete with outside jewelry stores renting space in the mall. There are certain behaviors that are kosher, and there are certain behaviors that destroy competition in unsavory ways. Ultimately, the illegality was not able to be shown, and Microsoft started a [not-so-good] trend of acquiring competition rather than dissuading its existence and adoption. This is in some ways a lot worse than finding them guilty.
It wasn't illegal if they had done it without having a monopoly share.
This seems to be getting a lot of downvotes, but I think it's correct, strictly speaking. There are a lot of business practices that are legal up until the point where you've acquired sufficient market power. After that, antitrust law applies. Similarly, regulators don't care if two companies with negligible market share want to merge, but they may care a lot if the two dominant players in a market want to merge.