Official support for Windows 98 ends July 12. In the meantime, Microsoft has decided not to releaes a patch for a critical flaw in Windows 98. This unpatched flaw could allow a hacker to take control of a users system.
It is common for a software developer to phase out software support for old software packages. If you are still running Windows 98 (snicker, snicker), you should really consider upgrading to a more recent Operating System. Most computers that run 98 will also run Windows 2000 and Windows 2000 is still supported and has already been patched.