The thing that makes the Windows timeline complex is that Windows has been developled from two totally different code bases.
The first versions of Windows (for home use) were really based on top of DOS and were 16 bit. These versions are Windows 1, 2, 3, 95, 98 and ME. This code base stopped at Windows ME in about 2000.
But at around the 1993 time frame Microsoft released a 32 bit operating system called Windows NT (New Technology) that was a totally new code base (not based on previous versions of Windows). At the time this was mainly for businesses use not home use.
Confusingly the first release of this was NT version 3.1, then it evolved into NT version 4, then Windows 2000.
Then Windows 2000 (merged with parts of Windows ME) was released as Windows XP (for Home AND business use), this then became Vista (after a major rewrite) and now Windows 7.
But I still dont know where they got "Windows 7" from, maybe they thought it just sounded good.
Now hopefully the chart on the web site below is a bit clearer
http://en.wikipedia.o..._of_Microsoft_Windows