Windows turned 25

lol, not really, the name makes sense as far as MS are concerned.

It follows like this :

1 - Windows 1.0
2 - Windows 2.0
3 - Windows 3.0, 3.1, NT 3.1, NT 3.5 (All version 3)
4 - Windows 95 was V 4.0 which ran parallel to NT 4.0, Win 98 was 4.10.1, Win ME was 4.10.2 (All version 4)
5 - Windows 2000 was V 5.0 and XP was 5.1 (All version 5 and first consumer based NT)
6 - Windows Vista is Version 6.0
7 - Windows 7 - The 7th release of windows