Years 2000 - 2010
1999 - Windows 98SE (Second Edition)
Windows 98SE is an update to Windows 98. This version fixed bugs found in the first Windows 98 version.
2000 - Windows 2000
This version was used more in the business market, but many home users used it as well. Microsoft initially intended for Windows 2000 to replace Windows 98, but soon after realized a whole new operating system was needed.
2000 - Windows ME (Millennium Edition)
This version of Windows was widely unpopular. It was made mostly for home users, and had some entertainment features not on previous versions of Windows. It was known to be unstable and was on the market for less than a year before Microsoft brought out the widely popular XP.
2001 - Windows XP
Up until this time, Windows XP was by far the most popular version of Windows. This version stayed on the market for 6 years and is still used widely today. It sold over 400 million copies by October 2006.
2007 - Windows Vista
Windows Vista had many criticisms and quickly became unpopular. One of the major criticisms with this version of Windows is the constant warning prompts shown to users.
2009 - Windows 7
Windows 7 is the current major release of Windows. Features of Windows 7 include advanced search, faster boot time, device stage, and improved management.
For more information on Windows XP and Windows 7 visit http://www.brighthub.com/computing/windows-platform/articles/34988.aspx