I like 2000. It can stay on for weeks at a time. I haven't tried months, seasons or years yet though.
As much as I would like to badmouth Microsoft right now, I have to say that Windows XP Professional version (when legitimately purchased) works quite well. When I first plugged my computer into the internet, all I had to do was plug in the cable and surf. That easy. Unlike Win 98 or whatever where you have to install like 400 drivers and reboot about 50 times.
http://fun.sdinet.de/flash/games/metelev1_01.swf use this link when u get frustrated of your windhows B)