Effective January 16, 2004.
from MS:
After January 16, 2004, this product will be obsolete and assisted support will no longer be available from Microsoft.
http://www.microsoft.com/windows/lifecycle/desktop_LevelOne.aspx
AS google shows
http://www.google.com/press/zeitgeist_nov03.html
almost 1/4th of the internet userbase is still using Windows 98. Should a substantial win98 virus arise after that time, it could be catastrophic.