From the Microsoft Website:
Windows 95
Product Availability: August 15, 1995
Mainstream Support: August 15, 1995 - December 31, 2000
Extended Support: December 31, 2000 - December 31, 2001 (This product has reached the end of its life cycle; no support is available.)
Upgrade Information: http://www.microsoft.com/windowsxp/pro/howtobuy/default.asp
Windows 98
Product Availability: June 30, 1998
Mainstream Support: June 30, 1998 - June 30, 2002
Extended Support: June 30, 2002 - January 16, 2004 (Extended hotfix support ends June 30, 2003. After January 16, 2004, this product will be obsolete and assisted support will no longer be available from Microsoft. Online self-help support will continue to be available until at least June 30, 2006.)
Upgrade Information: http://www.microsoft.com/windowsxp/pro/howtobuy/default.asp
Windows 98 Second Edition
Product Availability: June 30, 1999
Mainstream Support: June 30, 1999 - June 30, 2002
Extended Support: June 30, 2002 - January 16, 2004 (Extended hotfix support ends June 30, 2003. After January 16, 2004, this product will be obsolete and assisted support will no longer be available from Microsoft. Online self-help support will continue to be available until at least June 30, 2006.)
Upgrade Information: http://www.microsoft.com/windowsxp/pro/howtobuy/default.asp
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Legend
Product Availability: The duration of the product's availability.
Mainstream Support: The period of availability for standard support offerings (e.g., no-charge incident support, paid incident support, support charged on an hourly basis, support for warranty claims, hotfix support, and online self-help support information).
Extended Support: An extended period of assisted support, which may be charged on an hourly basis and can include hotfix support. Microsoft will not accept requests for warranty support, design changes, or new features during the extended phase.
Note that Win98SE is already nearing the end of it's Extended Support phase. Hotfixes are on a "when we have time" basis already.