Cost should be a consideration, too. I've used Windows computers for years for two reasons. First, windows hardware tends to be inexpensive and that's great because I prefer laptops and tend to destroy them in three years or less as I use them for everything from laying out magazines with Adobe InDesign to dealing with spreadsheets and maintaining Websites. In other words, I use a computer as an integral part of what I do for a living and I tend to wear out hardware quickly.
Second, everything in the world runs on Windows. There is no shortage of software when it comes to Windows machines.
A good alternative to Windows is Linux. It's a free operating system and each distribution of it has plenty of support through Internet forums full of Linux enthusiasts. Also, most software available for it is free.
Mac OS is a great choice for people who want computers that are easy to use and durable. You pay a premium for Macintosh hardware, however, and some programs you want might not be available for the platform. Ironically, a lot of Mac users run Windows in a virtual machine in order to run some software they need.