MS DOS, Windows 3.xx, Windows 95 and Windows NT 3.5x
Then I would get it before December 31st of this year, if I was you, because after that you won't be getting any. Yes, the above are now officially dead products. Or at least they will be when 2003 dawns.
The deadline for Windows NT 4.0 is still the end of 2004, and even although I couldn't find any official confirmation, I would imagine that Windows 98 will have the same deadline.