The only reason that Windows is more "unstable" than other operating systems is because is the most widely used, thus more hackers target it. It's the same with Firefox and Internet Explorer, Firefox has less security holes because they are not sought out as much as Internet Explorers.
By the way, Linux/Unix is for the most part, a server OS and is not meant for the average user, so basing Windows on Linux/Unix is quite stupid, seeing as how Windows would lose the majority of it's constumers due to the fact that most of them wouldn't know how to use Linux/Unix if presented to them.
Have you ever wondered how on Earth free Operating Systems are less widely used than expensive ones like Windows? The answer is simple, Windows is built for the computer illiterate consumer, it's not hard to use, and most people don't even realize, and frankly, don't care about the security risks involved with using Windows.