I was going to post on here singing Win2000's praises... only just tonight I was minding my own business when the machine reset and when it restarted NTOSKRNL.EXE was missing!! :-o Actually I suspect a trojan, I've been messing with a lot of freeware from strange places recently and I think I got stung :-(
Win2000 is the only Microsoft OS I've really been happy with for any length of time. Otherwise there's:
AmigaOS - i really wish a bulls**t-free OS like this will become popular once more
Mandrake Linux 9.1 - first distro I could really stand :-D
ZX-Basic ;-)