Actually, I still get a lot of use out of my old Windows boxes (I have a few!). Like a lot of other posters, I have turned some of them into Linux boxes. Arch Linux is my favorite, but I hear that Yoper 3.0 has just been released. Yoper 2.1 was hands down the fastest operating system I have ever used. I will investigate Yoper 3.0. All reports I have read say that it is like Yoper 2.1 - a bit rough around the edges, so don't go there if you aren't comfortable hacking around with Linux configs a bit - but equally fast: a real barn burner.
So, Linux for sure. I have also been going retro. I can't stand software bloat, and have been investigating how usable older Windows releases are these days. I have spent a fair amount of time configuring and playing with Windows 98SE (one of Redmond's best products IMHO) and lately, Windows ME. Windows ME, variously called "Mistake Edition", "Miserable Edition" and the like, and also labeled by PC World magazine as one of the top five worst technical products of all time, is actually pretty good if you do a clean install.
It is interesting because you get a relatively modern OS (remember, XP came out in 2001 and still chugs along) that requires only 32 MB of RAM and about 512 MB of disk! On an even reasonably modern computer, this system just screams along.
As you can tell, I am a speed freak - I want my machines to be fast and responsive to the touch. So, I use my old Windows boxes for stupid stuff like seeing how fast I can make them go.
I use my Mac for EVERYTHING else! I *live* on my Mac; I *tinker* with my PCs. That says it all to me. The Mac is rock solid, fast, reliable - it really does just work. The PCs take endless tinkering, but I am a techo-geek and sort of enjoy that!
My two cents.