If you want a free virus scanner for Linux, there is Clam AV. It comes on most Linux live CDs because they are used for rescuing computers that have been compromised. It's cross-platform (to my knowledge) and you can download it for free. Nessus is another free software program that is used to scan for web security threats. I don't know much about Ubuntu, but in Debian, I can install software fairly easily through the Synaptic package manager. I think Ubuntu has something equivalent to that, since it's based on Debian.
_________________
I don't want a good life. I want an interesting one.