Since there appears to be some lack of knowledge here I've decided to address the question of how M$'s monopoly came about.
The M$ situation is a little bit special. The growth of the PC clone market was so extensive that I don't think even Billy Boy was aware of just how big it was going to be. The flood of competitive pricing of bare hardware that could all run the same software but came with no software or support, even though they used more expensive builds than the comsumer oriented "home computers", eventually wiped the floor. Though the Mac did just managed to hold on to it's DTP perch long enough for the music industry to leave a gaping hole for Apple to slot Itunes and the Ipod into.
This was caused by IBM letting the clones be sold back into the US market. This could have been any other computer of the time, they were all being cloned in the eighties. I've heard that IBM had no choice, but who knows, maybe IBM wanted to see what would happen. After all, the PC was very small fry for IBM at the time. And considered to be a pretty basic design.
Initially, back in the 1980's, since this position was indeed up on offer, M$ just wanted the position of being primary OS supplier for this computer, the IBM PC, knowing full well how to exploit it. Billy achieved this goal (Probably his one real claim to fame) with some smart cut-throat bussiness practises and the rest is history as they say ...
Now there is a subtle edge to this situation, the PC became chiefly an end user product. This means it had to run "out of the box" and every additional tool for it also works out of the box. These end users don't know the first thing about compiling source code and matching versions and making scripts to clean up temparary files and debugging incompatibilities and ... nor do they want to deal with that. This creates a situation where these end users want the same setup that is used by everyone else.
In theory, different vendors could comply with defined API's. But free market competition natually encourages incompatibilies by design to be built-in. Even when the user demands otherwise. There has to be a serious financial advantange to comply.
So, in the end, there can be only one.
There is many reasons that a monopoly is bad but I'll leave that one for wikipedia.
Evan