One can violate antitrust laws without being a monopoly. Certain parts of the law (regarding collusion on price setting, for example) can be broken by very small businesses.

See this helpful FTC page: https://www.ftc.gov/tips-advice/competition-guidance/guide-a...

This is true, but those are all cases in which putative competitors collude to essentially form a cartel. Which is a distinct category of antitrust offenses from anti-competitive behavior.

As others note you don't need to be a monopoly to violate anti-trust laws. However, as it relates to being defined as a monopoly this ability to leverage your market position to stifle competition is the exact type of behavior that would support a finding of monopoly...most non-monopolies can't leverage their market position to unfairly compete

Isn't the important factor whether it's trying to become one? Windows and IE were never the only possible options.

A de facto monopoly doesn't mean that other options don't exist. Microsoft had a monopoly on the PC operating system market, despite other options existing.