Microsoft unveils the pre-beta release of Windows7

Windows7 (formerly codenamed Blackcomb and Vienna) is the next version of Microsoft Windows and the successor to Windows Vista.

Microsoft has stated that it is "scoping Windows7 development to a three-year timeframe", and that "the specific release date will ultimately be determined by meeting the quality bar."
According to Mike Nash, Microsoft’s vice-president of Windows product management, “The decision to use the name Windows7 is about simplicity. Simply put, this is the seventh release of Windows, so therefore Windows7 just makes sense.”

There have been far more than 7 releases of Windows. In saying "this is the seventh release of Windows"; it is the versions built on the NT kernel being counted.