There are some theories that if anyone found the true meaning of the universe, it would vanish and be replaced with something far more complex and confusing. There is evidence that this has already occured. -paraphrasing from Douglas Adams's Hitchhiker's Guide to the Galaxy
What would happen? Fairly simple. Certainly, the WINE project would be finished at an astonishing rate. People who still want to stick with Windows would enjoy numerous bug fixes floating around the net so that their OS of choice would be relatively stable for once. Companies that have formerly struggled with Microsoft's ability to integrate anything they do with Windows would find themselves able to compete again. Conversely, people who would rather do without all of Microsoft's 'convenient' integration would now be able to do so easily.
But for how many versions?
Certainly, after the entire source code fiasco was over, what would Microsoft do? Just concede defeat to the forces of openness and thereafter release all their versions of Windows as open source? Of course not. Part of Microsoft's success has been their deathgrip tight hold of their source code so that nobody can see how their system works so they can maintain a dictator-like control over everything that happens after Brian Eno's infamous 'Microsoft Sound' plays. They would lock down their code even harder than before.
Of course technology experts and code geeks would say that Microsoft's coding is sloppy and ameturish1. Of course instant compatibility with anything Windows-based would be avaliable for anything not Windows-based. But, when was the last time business owners and J. Random AOLler listened to actual technology experts or code geeks? They listen to the biggest guy with the flashiest ads, and in this case, that guy is Microsoft. In fact, Microsoft may throw around sufficient money and media clout to prevent the experts and geeks from even being heard.
So what would Microsoft do? They'd release another version of Windows quickly thereafter. And Microsoft's source is closed source, of course, of course. But, they'd release it with enough differences and incompatibilities between the most recent version of Windows (in this case, the now open source version) to make porting things very difficult again. And they would make shady deals with hardware manufacturers to have exclusive rights to certain technologies (Say, in theory, USB 3.0) so only their new Windows would run it. And, of course, they would certainly throw in enough flashy 'features' that users simply 'have' to own.
Now, you ask, who would be stupid enough to buy this, what with perfectly running clones of Windows running rampant in the world? The answer is: EVERYONE2. Remember what I said about business owners and J. Random AOLler listening to the biggest guy? For the longest of times, for better or worse, they've trusted the one biggest guy, and that's Microsoft. Even in light of better3 operating systems, Microsoft still maintains a stranglehold over the OS world. More and more people are using alternative operating systems, but not a sudden shift of millions upon millions of users. Many still trust Microsoft to do the Right Thing. And they wouldn't see the logic behind making something open source. All they'd need to see is the phrase "Microsoft has released a new version of Windows", and they'd buy it.
With tighter locks on the source code so that it isn't released to the public THIS time, Microsoft would restrict the people who know the code to even FEWER people, resulting in less eyes scrutinizing the code for bugs, resulting in more bugs. But, it's all irrelevant anyway, everyone would buy it and keep Microsoft the monopoly they are. And in time, the 'perfect' Windows emulators would fade into legacy emulators as programmers program for the next, closed version of Windows, entirely incompatible with present Windows emulators.
The end result? Maybe a bit of turmoil, maybe a small hit on Microsoft, but in the end, they'd still be there, sucking people dry.
1: Going on the theory that anyone who HAS seen the source code says that it is very shoddy programming.
2: Well... okay, not everyone.
3: This is admittedly a judgement call. I do consider nearly anything that is not Windows to be better stability and reliability-wise than Windows. Note that I did not mention a specific alternative OS in this writeup.