Windows 7: the naming scheme is somewhat complicated

http://en.wikipedia.org/wiki/History_of_Microsoft_Windows#Product_progression

* Windows 1.0
* Windows 2.0
* Windows 3.0
* Windows 95 (Windows 4.0)
* Windows 98 (Windows 4.1)
* Windows Millennium Edition (Windows 4.9)
* Windows NT 3.1
* Windows NT 4.0 including up to Service Pack 6a
* Windows 2000 (Windows NT 5.0) including up to Service Pack 4
* Windows XP (Windows NT 5.1) including up to Service Pack 3
* Windows Server 2003 (Windows NT 5.2)
* Windows Home Server (Windows NT 5.2)
* Windows Vista (Windows NT 6.0) including up to Service Pack 2
* Windows Server 2008 (Windows NT 6.0) including up to Service Pack 2
* Windows 7 (Windows NT 6.1)