I had Win 98 and 98 SE, Win 2000, Win 95, Win 95 OSR 2, Win 3.1, before that DOS (but I dont know which), had also some atari with his os...
And a ZX Spectrum :p I had 4 of those but accidents happened all the time :D
About 10 years ago I had some very old computer. It run DOS, and It was my first computer. :D I think I have it still somewhere on the warehouse. :D
I still use XP because Vista is a disaster, but I installed Windows 7 on my laptop (which is kinda slow) and it's definitely as fast as XP. I think I'll actually upgrade to Windows 7 when it comes out. There's nothing particularly exciting about Windows 7, but nothing that bugs me to death the way numerous Vista things do, so to keep up to date, I'd recommend it.