I was using Windows up until about 2/3 months ago (mainly xp, booting into 2k for testing/work stuff etc).
I fancied having a go at Linux, and liked the idea of just being able to install everything off of a net connection (well, nearly everything!), read up a wee bit on Gentoo then decided to try an install on my old p2 333 I use as backup storage (carefully removing the hd with the backups on, natch

).
One week later, when kde finally finished compiling, I liked it so much I now run all my boxes on Gentoo with testing/work being done in vmware installs of windows (oh how joyful the geek whose employer will pay for a vmware licence

!). I still unfortunately have to keep an xp install so I can play Homeworld 2 (and hl2, if it ever makes it to release

), but that's a small price to pay really, and I don't mind it (once more, hooray for employers who provide you with licences for outrageously-priced pieces of software

).
This isn't to say I haven't had any problems with it - I had a horrendous time breaking my RAID arrays with grub, but everything works now (more or less), and I wouldn't be without it.

Give a man a fire and he'll be warm for a day: set fire to him and he'll be warm for the rest of his life.