I won’t use a connected computer without a firewall…
You can try Kerio or Sygate or Outpost (three have free versions).
If you’re low on resources, you can use just the on-demmand (non-resident) scanning for Spy Sweeper and Adaware.
Oh, unistall NAV and be sure you follow the Symantec webpage info to remove all files and trash left behind…
Any malware that manages to get past your defences will have free reign to connect to the internet to either download more of the same, pass your personal data (user names, passwords, keylogger retrieved data, etc.) or open a backdoor to your computer, so outbound protection is essential.
I use LooknStop on one computer, but it is not free. On the laptop I use Kerio 2.1.5. It is free and I think can be found, although not supported. I have used it off and on for several years without problems.
There are some newer ones that are free as David mentioned. I read good things about Comodo, and I think it is still free. David and others here can give you better information, but I would not be without a firewall.
Yes, you do need a firewall on your 98 box…
But why use the newest version of ZA. I use an old version on my 98 box.
Anyway, much better than no firewall.