I have on machine that exists solely to use old hardware on and to serve mp3s out on my local net. I have another machine that is more of a workstation/server. it is a dual processor that runs a lot of scripts that automate tasks and it is where I test any serious programming that needs more power.
and I have my laptop that I do most things on like programming, some graphics, and financial stuff.
I used to have an extra box for linux stuff to do more database testing on, but it went away when one of my friends took it back to go to law school (someone that used to read this board, not sure if he still does).
my XP machines never crash - although the dual processor one used to crash a lot when I first installed it because it had a GeForce in it and its AGP controller didn't like the chipset of my dual processor machine.
the mp3 server is Win98se and it needs to be rebooted every two weeks or so due to the memory leaks that win98 has. (it doesn't run any programs, it soley exists as a file sharing box unless I want to scan something - but that program is closed most of the time - so the mem leak isn't an outside program).