As of October 22, 2003 the current supported versions of Windows appear to be 98, 98 SE, Millennium Edition, 2000 and XP. Support for 98 and 98 SE will end January 16, 2004. Support for Millennium Edition will end on December 31, 2004.
Support information for them can be found on the Internet using the following links:
For information on the support status of the version of Windows you are using, refer to Microsoft's Windows Life-Cycle Policy web page.
For information on the colourful history Windows, please refer to the Windows History page on Microsoft's web site.