Though Microsoft has gone back and forth for years on really putting Windows 98 and Windows ME into their "End of Life" clauses that will prevent them from updating the OS'es any further, it looks like this time they may have actually done it. A critical security vulnerability that lies within Windows 98, ME, 2000, XP and 2003 was recently discovered, and Microsoft has issued a fix for it. However, not for Windows 98/ME.
The vulnerability is easily avoided by having a firewalled machine, and indeed it's a strange person these days who would put a computer directly on the Internet, but many people still do. While some chastise Microsoft for not working to fix the flaw on the older systems, 98 is well over 7 years old and is long past what most companies support products for. If you use Windows 98/ME, you definitely should be behind a firewall of some sort. Keep yourself safe.