AT-Guard Firewall is one of the best security software for windows 98.
Program is quite simple to use with a little bit of knowledge about Internet.
Anyway you may come to some problems because of the large amount of rules you can give to AT-Guard. Each time some application from your computer tries to connect with the Internet or some application from remote computer tries to connect with your computer, AT-Guard will ask you whether you want to allow or block the connection, and whether you want to create rule for that specific application, computer and service. If you don't read those queries carefully you can easily block some of the applications or servers you use. Anyway, you can always use AT-Guard settings to change or delete rules you made.