The first time I used Linux was, funnily enough, to rescue files from my first Windows laptop. The hard drive had completely packed up, and by the time it managed to boot Windows, the whole thing would freeze up. I vaguely remembered that you could run some Linux distros from the CD, which might allow me to access the hard drive long enough to prise out the files I needed, and ended up doing just that with Knoppix.
A few years later, my parents ended up getting their hands on a laptop, but it didn't have an OS installed. I offered to stick Ubuntu on it for them (8.04 I think), and managed to get everything working except for the wireless card. Unfortunately they weren't big fans and went with Windows XP in the end, but I was intrigued enough to try dual booting my PC with it.
Since then I've always been dual booting Windows and some form of Linux. I really liked Ubuntu 10.04 and 10.10, and after discovering Mint I put Isadora on an old Pentium 4 laptop I use for playing music. I was really impressed with what it managed to achieve with Gnome, but Ubuntu still appealed to me more. I've just set up Julia on my desktop though, and after a bit of tweaking it's finally surpassed Ubuntu in my eyes as the best OS I've ever used. It runs like a dream, and with the addition of Docky and a spot of transparency for the gnome panel it looks stunning too. The main menu is just genius.
I actually rather like WIndows 7, but if it wasn't for the small matter of games I'd happily ditch it altogether for Mint 10.