You already are behind a firewall called iptables and have no open ports by default, a frontend for iptables is called "firestarter" as mentioned above. So there is no need for a huge install-button, since the firewall is already installed.
There is others that can explain this better than me i am sure, but the way Linux is set up with root (sudo) and user, a virus can not do much damage on your system unless it is written for Linux and you execute it yourself and give it clearance with your password. There is malicious code , see: http://ubuntuforums.org/announcement.php?f=13
,but that is also something you have to put in yourself.
As Linux grows more popular there will probaly be more viruses for it, but i think the XP way of getting a virus by visiting a website or opening a mail is nothing you have to worry about.
I would not loose any sleep over viruses if i were you, give Linux some more time and you will forget all about it (until you hear friends at work discussing wich virusprograms is the best and think: oh yes i used to be there;-))