Just found the (rather weak and boring) answer:

It is the major version number:

Winders 3.x = 3
95 / 98 / ME = 4
XP / 2000 = 5
Vista = 6

However, just to confuse everyone it is technically Windows 6.1.

Zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

Posted By: Steve in Holland, Jan 6, 13:54:26