reportedly) says that because of security, it is replacing Windows desktops with Macintoshes and Linux computers. Microsoft replies, claiming that Windows is the most secure operating system. Both are right.
Microsoft does a great job with cybersecurity. They have the largest and best team of security experts in the world. They employ more security experts than the next 100 top companies combined. If you combined all the experts from the anti-virus companies with all the experts from the military and NSA, you would still only have a small percentage of the expertise at Microsoft's command.
Windows 7 is probably the most secure operating system available, and Internet Explorer 8 (IE8) is probably the most secure web browser. Microsoft has done a great job of not only securing their products, but in ways that encourage people to make good security choices.
The problem is that none of this is enough. Microsoft is the biggest target. Hackers largely ignore Linux and Macintosh desktops - they focus on Windows. It doesn't matter how secure Windows is, or how insecure Linux/Macintosh are: as a practical matter, you are still more at risk running Windows.
Changing the desktop operating system is a practical solution. But I'm not claiming it's a smart move. If everybody did it, then hackers would just change tactics. I can never recommend it as a good national cybersecurity strategy, or even a good strategy for government desktops. It's just a good temporary strategy for individuals, for the near future.
Microsoft cannot fix stupid. The biggest security problems aren't in the operating system but in people's heads. People routinely misjudge risk and make poor decisions that get themselves hacked. People download and install software with reckless abandon. You can break into somebody's computer by forging an e-mail from their IT department claiming they need to install an urgent patch. Enough people will do this, installing the Trojan virus, that it becomes nearly impossible to secure a Windows desktop. Windows is the operating system "most vulnerable to viruses" not because of any feature in the operating system, but because its users are most like to stupidly infect themselves.
Security is a trade-off. There are not a lot of solutions to these problems that don't cause more problems. For example, you can stop users from infecting themselves with viruses by preventing them from installing any software at all. However, this means that they can't install software they need - so they end up working on their personal laptops, which are probably infected with multiple viruses already.
The question is quickly becoming moot anyway. More and more activity is moving to the cloud and mobile devices. In a couple of years, the majority of cybersecurity threats you will face will not depend on your choice of desktop operating system. Then, of course, everyone will be hating monopolies like Google and Apple instead of hating Microsoft.
APPENDIX: People are most likely to dispute my glowing description of Microsoft. This is based on my personal dealings with Microsoft, as well as hacking Windows. It's not based on third hand information.