Microsoft has proven in the past that it was insecure.

When the source code to XP leaked, there soon were all kinds of worms and trojans. Microsoft relies on security through obscurity, which for some people isn't obscure at all.

If you have the reverse engineering tools, probably custom made, by some foreign investor, you could peak through the security measures. Also microsoft relies on fast patching for problems that are often circumvented sooner or later, because of security vulnerabilities in the patches. I don't need to say, that a patch is easier to reverse than an OS, and presumably quite revealing.

I feel I should point out once again that if Apple or Linux was the #1 most popular with like a 75%+ market share, they'd be the horribly insecure ones that are getting hacked all the time. It's not about the product quality, it's about what thousands of foreign programmers are targetting because they're going to find a security hole eventually no matter what system it is.