As a programmer and a person very much in favor of open source software and such geeky causes, I was very torn on what operating system to install when I upgraded my computer. On one hand, I had any of the many flavors of Linux. On the other hand, I had the Microsoft Windows family.
Linux was the obvious choice in many ways. You can't beat the price point of being totally free. It has come a long way from the last time I ran it on my personal computer in terms of usability, hardware support, and diversity of applications.
The Windows family of course has numerous drawbacks. It's expensive, closed source, and bloated (not that Linux isn't a bit bloated at this point). Of course the biggest pro in favor of Windows is simply that all applications work on Windows.
In the end, my decision had to come down to the one deciding factor of making sure everything just worked. One of my friends wants to develop a small game on the side at home and some of the possibilities came down to Windows only tools. For example, Microsoft released an API to develop Xbox Live games for home users. I'd bet the odds are that it would take a lot of effort to get it to work on Linux, if it's possible at all.
After giving in to the dark side, I was left with choosing a version of Windows. As much as I wanted to believe that Windows Vista might be a viable option, in the end, I had to choose Windows XP over Vista. It's simply much more efficient.
Additionally, I wanted to move to a 64-bit operating system because computers are now hitting the 4 GB address space limitation of 32-bit operating systems. Plus, it's a shame that the 64-bit processing capabilities of my last computer were completely ignored because I still ran a 32-bit operating system.
That's how I settled on Windows XP 64. I'm not all that pleased with still running Windows, but I do have to admit that my computer is fast and rock solid. I did have to blow $150 to get a legal license for it, but I don't regret it yet.