There are two types of people in the world: PCs and Macs. And then there are those who appreciate both. I have a lot of reasons why I use both operating systems, and here are a few.
Games, obviously is the first reason I run windows. Actually, it is really the only reason I run windows to be honest. I built myself a PC simply for gaming purposes. Although I do like word processing much better on the PC, I also have more options for accessories and looks. For example, you can't really get a Mac that lights up blue and purple and has a touch screen adjustable keyboard. Also, when something goes wrong on my PC I can just open it up and fix it, the software is also easier to manipulate. Which is also a downside. Viruses ew! I am primarily a PC gamer though, so I hold some fierce protectiveness about my operating system.
Mac
When i'm not blowing things up, I am a creative person. This is one reason I love macs. Other than the fact that my college runs entirely on mac in my department, the options for music edition software and other creative software is much larger. Pro tools and Logic Pro both run better on a Mac. Macs are streamlined, prettier, and because all of the parts were made by the same company and were made to go together, you have less funny little bugs you have to work out. They are also grossly expensive and hard to fix. Sometimes the operating system is so dumbed down that it is hard to fix problems unless you are really well versed in Mac.
My next goal: Linux.