My main OS was Windows XP. I started messing around with Damn Small Linux on a bootable CD-R. I didn't do it for any other reason other than curiosity. It was really intriguing to me that I could boot into a functional operating system with a bunch of decent tools, without having to install anything. I think I also messed around with Puppy Linux, I can't recall. From there, my interest in Linux increased and I went on to try a full distro, Ubuntu (I think it was version 6). Again, it was all about curiosity, and I was just playing around with it instead of using it as a replacement for Windows.
Sometime in 2009 I received an old IBM Thinkpad T30 for free from a friend, and decided I would only install Linux on it, instead of any flavor of Windows. I decided to go with Xubuntu, because after shortly messing around with Gnome, KDE, and XFCE, I decided XFCE was best suited to my preferences. I used Xubuntu for a couple years, and greatly enjoyed the experience.
In 2011 I decided to try some different distros, just to see what else was out there. I shortly tried Fedora and OpenSUSE, and decided I didn't really like them. Then I tried Mint, and fell in love. Mint, on the surface at least, seems to have much better driver support than any distro I had used previously. Maybe it's because they use some "non-free" / "closed" software or whatever, but honestly, the philosophy doesn't really matter to me as a user. Everything just seems to work, and the update manager works great as well. It comes packaged with an awesome selection of software from the get go, and configuration of any type was really minimal. I still use Mint 10 on my laptop to this day. It hasn't replaced Windows 7 on my desktop, but it honestly would, if I could play all of my games.