Microsoft has ended customer incident support or technical support and no longer issues security updates or patches for Windows 98 and Windows ME (Millennium Edition) operating system as part of its product lifecycle policy, as both OSes have reached the point of architectural obsolescence as they’re developed before the era of sophisticated internet viruses. That would means that the OS users will further expose to new security threats and risks.
However, if you decided to continue using Windows 98 or Windows ME,
Washington Post (dead link) has tested a few freeware and trialware of various security tools including anti-virus software and firewall that install, runs and uninstall properly on Windows 98 and Windows ME without causing Blue Screen of Death (BSOD). But before that, you will need to download, install and update Internet Explorer to Internet Explorer 7 or Internet Explorer 8 which is more secured.
Here is the list of security tools that supports Windows 98 and Windows ME:
Anti Virus Software
EZTrust Anti-Virus (one year free usage) (offer no longer available), F-Secure Antivirus 2006 (six-month free trial) (offer no longer available), Kaspersky Anti-Virus 6.0 (30-day trial), Panda Platinum 2006 Internet Security (90-day trial) (offer no longer available), McAfee Virus Scan (90-day free trial) (offer no longer available), Trend Micro PC-cillin Internet Security 2006 (90-day trial) (offer no longer available), F-Prot Antivirus (30-day trial).
Firewall Protection Software
Jetico Personal Firewall (no longer free), Tiny Personal Firewall (acquired by CA), ZoneAlarm prior to version 5.5 (discontinued).
eTrust PestPatrol (30-day trial) (no longer available), McAfee Anti-Spyware (30-day trial) (no longer available), Sunbelt Software’s Counterspy (15-day trial) (no longer available)