Most people are running Windows on their computers. In large part because Windows was installed on the computer when they bought it. Most people have never used any operating system that is not a Microsoft product. From DOS to Windows 7. Is this because Microsoft makes a great product? I certainly don't think that is the case. Give Microsoft credit for marketing practices. They are great at marketing. The marketing has always exceeded the product.
Windows is pretty much a barebones system. In other words, people must rely on third party software. Especially for improved security.
Power users can use Windows safely and effectively. However, in my opinion the average computer user does not know how to secure a Windows system.
The less a person knows about computers the better off they would be with one of several Linux systems in many if not most cases in my opinion. Linux isn't just for Geeks anymore. Those days are gone. Today there are a number of user friendly Linux distributions well suited for beginners.
Most people use a computer for email, to surf the web, manage photos, listen to mp3's or other music formats, video, messaging, word processing and etc.. Most people can do everything in Linux that they do in Windows.
In Linux one does not need to worry about viruses, spyware, cleaning the registry, de- fragmenting disks and running frequent scans. The last time I checked, just over a year ago, there were more than 2,000,000 pieces of malware designed to attack Windows systems. That number was and is growing exponentially. The last time I checked there were 42 Linux viruses, trojans and worms with most all of them designed to attack Linux Servers not Desktop users. The viruses pose a minimal threat to the secure Linux system.