There is not a lot of free security software available for Windows 98 these days since many software companies stop supporting software for Windows 98 and ME since only a small percentage of computers use Windows 98 or ME. But, if you are in the small percentage, you still need an antivirus, antispyware, antimalware, and firewall. Plus, buying a newer version of Windows for your old Windows 98 computer is not worth it in many cases since most Windows 98 and ME computers will probably run Windows XP very slow, or the computer does not run well at all.
You can also install a free Linux distro on your Windows 98 computer, but it can be time consuming to install and learn to use Linux. I recommend earlier versions of Ubuntu like Ubuntu 7 or 8. Kubuntu and Xubuntu might be good for your needs as well.
Avast 4.8  is a great antivirus for Windows 98. It does not use a lot of system resources, and its scans are pretty fast. It kept my Windows 98 computer virus free for many years.   (Avast seems to discontinued 4.8), so it will only protect you from old viruses.
ClamWin is a good opensource virus scanner which works on Windows 98 for scanning your computer with ClamAV’s opensource virus definitions which are updated automatically on a daily basis. ClamWin does not have an realtime shield, so you need to use Clam Sentinel which is a third party plug-in/add-on for ClamWin which gives ClamWin an active shield which detects viruses with the ClamWin virus databases.
Spybot – Search and Destroy is a excellent antispyware program which works with Windows 98. The last time I scanned my Windows 98 computer. It found 46 spyware infections and remove them all. It also has an immunization function which protects your computer from visiting bad websites on the Internet. The Immunization function uses no  system resources.
Spywareblaster is similar to Spybot’s Immunization function, and it also use no system resources while preventing your computer from visiting spyware infected websites.
Emsisoft Anti-Malware 5.0 is an Antimalware which has an Windows 98 version. It helped me removed two pieces of spyware although it is only free for 30 days because it is trialware.
Sygate Firewall was a pretty good firewall when I used it in Windows 98, but these days I just use the Firewall on my Router to save system Resources. Sadly, it was bought by Symantec and they discontinued the consumer software firewall, but you can Find SyGate firewall software online pretty easily.
A router for your home network has a built-in firewall which stops many hackers, and worms from attacking your computer.
Clear Cloud DNS is a DNS server which can stop you from visiting virus and spyware infected websites which SunBelt software who is the maker’s of VIPRE Antivirus. You can just configure your Network Adapter in Windows 98 to use Clear Cloud DNS address, or your can use Clear Cloud DNS address on your router control panel.
You can always boot from Ultimate Boot CD for Windows and scan your computer with UBCD for Windows security tools to remove viruses, but the definition files are outdated, so you should update them.
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nice post !! im using zenok Free Antivirus i think is good i have in my computer for two years ago . thanks for sharing 🙂
I been using Panda Cloud in Windows 7, and it is very lightweight, and does not slow down my PC like other antiviruses which rely on definition files which need to be downloaded from the internet. I check out Zenok out. Thanks for the recommendation.