Explain to me why windows is not the most secure OS? Because it is, from a standpoint of the OS's built-in defenses. Windows has next to no known vulnerabilities inherent to the OS, as opposed to OS X and Linux. This of course is counter-balanced by the fact that there, like I said before, are a ton of viruses and different kinds exploits out there, many of which run through third party programs (e.g. java vulnerabilities) that have nothing to do with windows itself. From a software standpoint, windows is securer than both OSX and the various Linux distros out there. Reason why those "get less viruses" is STRICTLY down to the fact that there's a fraction of viruses even available for these platforms, if microsoft went unix, all the viruses would subsequently - and obviously - be coded to infect unix-based systems. (People who code viruses overwhelmingly code for the platform with the largest market share)
"W7 is still windows". "Apple does Unix"
So? You seem inherently convinced of the fact that a unix-based OS is a better OS, you haven't told me why yet, though. Security? Like I said before, no. If there were as many viruses available for mac OS as there already are for windows, you'd need an anti-virus software for your mac, too. Doesn't fragment? OS X fragments too. (Just one link, but really, google it, you'll find many more testimonials to the fact.
https://discussions.apple.com/docs/DOC-4032)
Slowdown: See next point:
"Defragging makes it run faster"
Apart from the fact that defragging can make OS X run faster too because OS X *does* fragment files, in the case of windows the culprit for slowdown is mostly the fact that windows gives a downloaded application full access to every aspect of the operating system, meaning once you've agreed to let windows run the application, it can essentially do what it wants to do. This is largely to blame for the major slowdown, since users who download low quality bloat / shovelware 3rd party apps which create dozens of entries in the registry, shart all over your HD and auto-run, slowing the entire operating system down. It takes some computer literacy to keep this from happening, though again, it's not an inherent flaw, and microsoft could reduce the area of control a single application has, just like OS X does, without switching out their OS / filesystem.
There is simply no real need for Microsoft to switch, the few things microsoft can be faulted for are easily eliminated through simply tweaking the way it handles application permissions. (Note: OS X handles app permissions restrictively, this is not an inherent feature of unix though, it's simply a feature apple added to their OS)
However, the reasons NOT to switch to unix are obvious: believe it or not, but most fortune 500 companies, as well as countless others run windows computers, most of these value backward compatibility, microsoft would risk losing billions in revenue and temporarily crippling productivity on a massive scale, as everything would have to be essentially re-written and re-formatted for a different OS. As for eliminating the backward compatibility on consumer versions, yes, I agree they should do that, though even there plenty of private users value that, too. Switching anything at this point would be akin to shooting themselves in the knee, and switching to unix would just be stupid because, since, as aforementioned a.) unix DOES fragment, and b.) it is NOT more secure, and gets viruses just as easily as any other operating system, the benefit to the user is pretty much non-existent.
What microsoft should do, though:
a.) Introduce a version without backwards compatibility for those who don't need it (even though technically, backwards compatibility doesn't exactly matter to the average user, and won't make a difference in everyday use for them, if or not it is present), and;
b.) Introduce at least the option of restricting the permissions that applications have, even after being allowed to run, a.k.a. to save people from their own stupidity, like OS X does.
None of this has anything to do with unix itself so again, I don't understand why you believe Microsoft using unix would change anything, but I digress.