It's funny: I use windows on my "real" computer, and I really just experiment with linux on my other old computer(s), yet I get branded as a Linux Fanboy whenever I mention open-source in most parts of the Internet. Conversely, I get branded as a Windows-moron whenever I pull out my laptop at school where I use Linux exclusively.
I'll argue both sides:
Windows is useful, because the software base is quite large and the fact that it has billions of users means that 90% of software not only runs on it, but is actually written exclusively for it.
The newest versions may be bloated and only fit to run on the newest computers, but you wouldn't try to run the newest linux distro on your old Pentium two either, now would you? (Unless it was designed for that purpose)
Linux, and open-source in general, has many major benefits. I would like to see more people considering open-source software as an alternative to expensive software that does the same thing. Free as in speech software gets developed very fast and bugs get sorted out in realtime, so rather than waiting seven years for an update, you can get an improved version of what you've been using tomorrow if you want. Free as in beer software, of course, has the massive benefit of not costing any money. What's the worst that can happen? If you don't like it, don't use it! I'll try anything free (and not obviously virus-ridden) at least once, just in case I find a real jewel.
I've never had a security breach on any of my computers, so I won't discuss the security of the various operating systems here. All you need to do is practice safe exploration of the series of tubes!
I only use Linux but sadly I have to point out a few perks that Windows has that you missed, backwards compability maybe not the oldest DOS programs but most of the old crap does work and aiming at break free updates (this is the reason their updates take more time and their OS is somewhat bloated to start with *not counting Vistas poor attempt to have a "3D desktop features etc"*, you can't get 100% of that but at least they try), on Linux a simple update of one library file could result in having to recompile everything or break key features and so on.
But biggest of them all, the mother of all reasons why Windows will remain number one until Linux gets it as well, Universal standards! Given that Windows doesn't have as many variations as Linux, that is one reason why Linux is better, you can pick the one that suits you, but it's also the curse that haunts Linux.
Until there is a standard that stretches over "all" distributions or one type of dist. that takes over, Linux will never be able to compete with Windows on a big market, there is too much fighting among the dists. them selfs and there for not many commercial software devs will move to Linux (even tho some really want to) and the level of software won't reach the same heights as for Windows.
But I still prefer Linux ^^