Started in DOS, went to Windows (futzed with OS/2), and I loved Windows 2000. Around that time, spyware/malware was a big deal, and commercial apps began shipping with such apps bundled in. As much as I tried maintaining my Windows install, I would eventually have to reinstall, hunt for drivers (big pain getting SBLive! and ATI TV Tuner @ 640x480 to work properly, had to patch registers to stop audio popping), dig out serial numbers, dig out old setup.exe of the software versions I liked before they changed something I didn't like, it was a pain in the ass.
I didn't want to move to XP and get more of this. I stayed on 2000 as long as possible. Most apps I was using were open source, because I started moving away from commercial apps which were becoming increasingly annoying to deal with because of serial numbers, updates, adware, and around that time it was a fad to create Windows software that didn't use standard GUI elements, and I like using the keyboard to navigate.
Eventually the only thing I was using commercial software for was just the OS, so I moved to Linux. I wasn't too worried about games because I found those annoying even in Windows, having to download updates and patches just to get games to work properly, having to have specific driver versions and setting up my gamepad and using joy2key etc was annoying to me. Having to hunt down my save files when I reinstalled was a pain too, this was before they decided to put them in My Documents. I prefer consoles for gaming and I'm happy to have them just work when I pop in a disc.
Eventually I was in the market for a laptop so I bought Apple for the battery life, features, and durability. I liked OS X and used it for a while, but as OS updates would change the behavior of Finder or other things like that, it would annoy me. I also don't like the way certain apps need to launch X11 and can't use OS X's native GUI as well, and I was already happy with Linux so I just put that on.