>There were stories about how they used to deliberately tweak the OS to break competitor software.
The reverse, as far as I know:
http://blogs.msdn.com/b/oldnewthing/archive/2003/10/15/55296...
This is just the tip of the iceberg with respect to application compatibility. I could probably write for months solely about bad things apps do and what we had to do to get them to work again (often in spite of themselves). Which is why I get particularly furious when people accuse Microsoft of maliciously breaking applications during OS upgrades. If any application failed to run on Windows 95, I took it as a personal failure. I spent many sleepless nights fixing bugs in third-party programs just so they could keep running on Windows 95.
That's awful. But I think writing a competing app is a completely proper way to compete.
> they really did have a strong monopoly
Evidently they did not.
To those of us old enough to remember the "unbreakable" IBM monopoly, it's hard to worry too much about this stuff.