RE: How do I count thee?
At a guess....
Windows 9: 3.x, 95 (4), 98 (4.5), 2000 (5), ME (4.6), XP (5.5), Vista (6), "Vienna" (7).
Based on Codebase, i.e. jump to 32 bit dropping the non-nt kernel base, security re-writes as major version increases.
Saying that, the numbering scheme should really ignore 3.5 and 9X/ME as the current code base is from the NT4 tree, and hence uses that numbering scheme. It happens to co-incide well though.
(and yes, it too some jiggery-pokery to get the numbers sensible, then work backwords to what MS would think are reasons for a major version jump.
Question is, what will make & different enough from vists to be called a major version jump?