My main machine has Win2k Pro, my secondary is Win2k server and my dedicated server is running Win2k server.
The dedicated server won't see a reinstall of the OS for a LONG time -- Win2k is plenty stable. I only reinstalled Win2k on that box because I installed DX8 and it started to flake out (damn that requirement of DX8 for dedicated sound support, of all things).
The second computer doesn't get a reinstall that often (maybe once every two years). This machine tends to play games, but not much else, so its hardly touched.
As far as my main machine goes, I just reinstalled Win2k on it (from XP Pro). For some reason, XP just started to "hang" on certain things. Dunno why it started to happen, but Win2k didn't do it to me, so I switched back.
Generally speaking, my main machine sees a reinstall at least 2-3 times a year. I tend to get bored with the configuration often, so I'm always messing/tweaking the box. Usually, when I get a cpu/mobo upgrade, a new install is definitely in order. I'll be holding off on that until I get my hands on a 64bit processor (AMD's of course). Once I get that new chip/mobo/ram, 64 bit/66mhz cards, a full reinstall is in order.
vash