Win2K
I don't actually own this one, it belongs to The Company. I must admit though, even ol' 'Window's hating' rasterboy thinks this machine is OK. I've used NT 3.5 and 4 previously, and 2000 is actually an improvement. Oh don't worry, there are still a lot of things wrong with the user experience, otherwise it wouldn't be Windows! The nice thing is that I can have it running Apache, Perl, Zope, Python, PHP, and lots of other stuff that doesn't run easily or at all, on the Mac OS (classic, I'm sure OS X will have no problem.) Of course all of those things are much better experienced under *nix (like Mac OS X) but if you are stuck with a Windows machine, make the most of it...