if you're just using a linux box, you don't need antivirus, because practically speaking there aren't any viruses in linux.
the firewall in linux is iptables, and is implemented through the kernel. "easy to use" is not the first phrase that comes to mind when dealing with iptables, but there are front ends to make it easier, e.g.
guarddog and
firestarter. because you're using kde, i would recommend guarddog, although you could run firestarter if you had the gnome libraries on your system.
you could also find a basic iptables script somewhere to use for basic protection (i'd show you mine, but then i'd have to kill you
j/k), or there are places online where you can make a custom script, the url of which escapes me at the moment. there is -tons- of information on the web about iptables, though, so check your favorite search engine.
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