1. Windows 1.0
2. Windows 2.0
3. Windows 2.1x
4. Windows 3.0
5. Windows 3.1
6. Windows NT 3.5
7. Windows NT 3.51
8. Windows 95
9. Windows NT 4
10. Windows 98
11. Windows ME
12. Windows 2000
13. Windows XP
14. Windows Vista
15. Windows 7
they have two separate lines.
windows 1,2,3.0,3.1,3.11,95 (4.0),98(4.1.1998), 98SE (4.1.2222) and ME (4.90.3000) are based off the same codebase.
Windows NT 3.1, 3.5,3.51, 4.0, 5 (windows 2000) 5.1 (windows XP) and 6.0 (Windows Vista) are based off the NT codebase.
Version numbers don't transcend multiple codebases.
so in general what we have are 7 versions,of windows NT, before windows 7. however, since windows Vista is 6.0 windows 7 can easily be called version 7. (although in actuality I believe it's version number is 6.1)