Having used Windows from Ms-Dos until Windows XP, I've made my way through countless of Windows problems and errors.
a) I was tired of trying to fix spyware and malware issues. It had come to a point whereby restricting my surfing to only a limited number of sites, no longer worked. After all, even some of the "safest" internet sites can be hijacked by ads containing spyware and malware. And then what? I'd have to give up using the internet for good?
b) I used to be a real Windows fan. But one day, after having dealt with so many problems, I just couldn't take it anymore. Yet, it'd take years later for me to actually switch to Linux. From simple problems like missing icons to explorer crashing and to really serious issues like .doc opening in Winamp, .avi opening in Explorer, etc., I'd faced many of them. And the really worst issues would be Windows trying to do something annoying to critical system files.
But it wasn't a sole problem that broke the camel's back. It was dealing with years after years of problems in Windows that often sucked away a good part of my time, where I finally manifested a real dread for Windows. And that delight at Windows when I was still a child finally turned into bitterness at Windows as an adult.
c) Got really tired of Microsoft's behaviour.
I used to love Windows until it had its' problems. And I used to really like Microsoft until it turned out they were a bunch of scumbugs from the day MS was founded. That even the ideas for Internet Explorer were "stolen" from other companies. (If I'm not wrong, that is). And finally, one day, all that admiration at Bill Gates turned into pure annoyance.
d) I can't afford Vista.
Vista sounds cool but I can't afford it. I mean... that's a few thousand bucks! And all that for Aero which can't even compare to Beryl? Besides, going green is a huge issue right now.
There are many more reasons though but these are the ones I can think of right now.