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You dont need an anti-virus in Linux. Using Linux you are immune to every bug out there. You can uninstall clam and rest easy. I open .exe files and e-mails with impunity. Ubuntu also comes with a firewall so you dont need to add one.

Youre NOT immune against virus-like things, if - youre sharing binaries with mates, as you used to share win progs =). - youre not looking into source(usually done on open reps). - you dont know what sudo, root/user accounts are for. - do not update ur system regulary/ setup firewall. Linux is a collection of powertools and compilers. Remember this. As for antiviruses, they are usually set on linux to check windoze data. Yes, usually on mailservers etc. For firewall, theres a pretty strong network filtering built into Linux (iptables/ipchains). I recommend you to study it IN DETAIL. But for the first time, you can use Firestarter frontend. Not much flexibility, but its far more easier. To deactivate unneeded services, you can also use script "Bastille Linux". Of course that requires some knowlegde, but all things do same.