It depends whose side you want to hear. OSS is "free like the speac", not necessary as "free beer".
99.9% of OSS users do not care to look at the source, or to compile from the source, all they care about is to turn their computer ON, login, and check email / facebook / fix a photo / write some software.
When talking about Windows - don't forget that there's good deal of OSS which actually runs under Windows.
During the past seven years I've kept a dual-boot setup on my personal home computers... often multi-booting a few different Linux distros and a single Windows option because I enjoy trying new Linux versions while still keeping my current best-working version as the king until dethroned by something I find better (IMHO). But Windows is always an option for those things (games mostly) that can't be used on Linux, or things that have no Linux equivalent. Obviously, in my home, Windows has been relegated to second-class citizen and only serves as a sort of gaming console. The spirit of the Linux community is part of the whole grand experience of being part of something vibrant rather than being a piss-ant under the thumb of a money-motivated corporate entity who most often sides with other corporate entities and against the consumer/enthusiast.
Now add to the Windows experience the destructive jerks who create malware, sneakware, ransom-ware and deceptive traps which most often hurt the very innocent among us... and realize that with Linux I have never (yet) had to deal with that sort of crap.
There are plenty other pro's and cons to both systems... but for me, as a home computer-user, the above points are the reason I'm a Linux enthusiast and Windows-when-necessary guy.
Linux likes: Has been malware free (for friends that are prone to malware), easy to install most programs (software center), updates are not forced upon you, almost never a reboot when updates are applied, runs stable without reboot, the cost, can install many different desktop environments if you want to mix it up, mythtv once it's configured properly, nobody really cares about my information
Linux dislikes: Graphics drivers (quality and installation), if something goes wrong, it's not always easy (probably because my level of experience with Windows vs Linux is much greater), no support for many of the programs I use for work, lack of major games, mythtv configuration
Windows likes: Tons of programs and tons of games, graphics drivers are much easier to deal with, I like Visual Studio IDE, the default for hardware/software support is Windows, stability
Windows dislikes: Most updates require a reboot, you have to go pretty far out of your way to figure out what the update is for, their new found desire to mine your data, what should be simple drivers often are 200MB downloads with a ton of crap bundled with it (I'm talking about you printers!).
Choosing an OS for anyone isn't a very easy task. There're pros and cons to everything in life, and OSes is no different. I'm one of those few people that use many OSes on a daily basis, and as part of the open-source community I do look at, write, and review code from time to time. That's not to say that I look at the source of every single package I install; rather, I take a look at the code of that which I believe is necessary. Major open-source packages get reviewed, and when someone violates the trust of others their software is quickly flagged and removed. If the perpetrator isn't the maintainer, then, they're quickly blacklisted from other projects and their reputation goes to the crap bucket. One of the reasons why people don't add malicious code is because "we" (i.e., members of the open-source community) will hunt you down. We won't hack you because we operate within the law, but I guarantee that no matter how good you are at tunneling and hiding yourself, we'll find you and you will be brought to justice. This results in programmers losing their jobs and completely losing any reputation. So, for this reason, people don't intentionally try to do bad things when they know of the consequences (i.e., never getting hired as a software engineer again).
Now, Windows is as good as OS X, Linux, Unix, and other Unix-variants. Each OS is good for something, and each OS is better suited at certain tasks than another. If you want to know the most secure OS out-of-box, the answer is probably OpenBSD. If you want to know the best OS for gaming, it's definitely Windows. Or, if you want to know which OS has the most beautiful GUI, the answer is likely OS X. If you ask which OS is the best for embedded systems and frugal people, my answer is Linux.