95 = 4.0
98 = 4.1
ME = 4.9
2000 = 5.0
XP = 5.1
XP 64-bit/Server 2003 = 5.2
Vista/Server 2008 = 6.0
7/2008 R2 = 6.1
If we’re going to include 2000 because XP brought in the NT kernel then we should really drop 95, 98 and ME because they’re from the non-NT chain. So it would really be NT4, 2000 (5), XP as 5.x, Vista as 6.
Really it’s MS numbering, they just make crap up.