Don’t forget Service packs...
I don't count service packs as separate versions, although it's fair to say that some (such as XP-SP2) contained considerable new features and technology, and perhaps should be considered their own version. I try not to open that can-o'-worms argument.
For instance, was Win98SE a different version from Win98, because it included the first real working support for USB, arguably the most important technology to come to Windows since networking? Opinions differ, and YMMV.
On the other hand, some folks are calling Win7 "Vista-SP2" which I think is kinda funny, since compared to what Windows 7 was originally supposed to be, is pretty much true. Even Steve Ballmer described Win7 as merely an upgrade of Vista, rather than something significantly different (he was trying to deflect the incompatibility problems, and managed to admit that Win7 was Vista with a few things fixed, "Vista" crossed out, and "Win7" written in crayon).
So I don't get into whether service packs are versions. In this context, I don't count them.