Well I remember this question being asked very recently in the forums.
Currently avast even supports win95 and that went of MS's radar a considerable time ago, but I haven't heard any suggestion that avast will drop support for that or win98/ME, etc.
I wouldn't recommend any old AV solution, NAV or otherwise.
Yes you do need a firewall, there are others, Comodo, Jetico, Sunbelt Kerio, etc. you would have to check on their support for win98.
See some firewall tests for comparison, some are freeware but many are paid for versions
http://www.firewallleaktester.com/tests.php. Also see
http://www.thefreecountry.com/security/firewalls.shtmlAny 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.