Microsoft lost its position of dominance through purely market ends (in my opinion -- certainly between the US actions and the EU actions you could argue that they made a difference).

In retrospect breaking up MS between Windows and Office was not a necessary condition to breaking MS's monopoly, and so we have an excellent test case for government intervention at that scale. Not definitive, of course; MS could very well be the exception, but certainly the Justice Department's actions against Microsoft should not be used as evidence that such actions are effective.