Windows 98 and ME are legacy Operating Systems which are being maintained on extended support by Microsoft Security.  Security releases for 98 and ME will generally follow the security updates made for XP and 2000. 

Several critical updates for calendar year 2005 are now available through the Windows Update process for 98 and ME. In updating one of our home PCs, six critical updates were applied and everything is working well so far.  

Windows 98 and ME Security updates available under Windows Update
http://www.techweb.com/wire/security/159400240

  Windows Update Link for 98 and ME 
http://www.microsoft.com/windowsupdate