Firewall: iptables. You probably have it installed, you just need to learn howto configure it.
Antivirus: Linux isn't Windows. If you're running a pure (single-boot) Linux system with the ext2 or ext3 filesystem, and don't normally work as root, then there is little need for virus scanning. If you dual boot with Windows and run Wine, mount you Windows partition read-only. If Windows users store files on your Linux machine you might want a virus scanner for *their* benefit.