Firewall and antivirus

From what I read a "firewall" is not needed if Ubuntu is not specifically setup as a server. Please excuse my "windows mindset" but is this true?
I do notice my "cable-modem" lights are acting normal, no incoming-outgoing traffic as I write this (how great).

Same for anti-virus. Not needed ... what a strain on my Quote: "windows mindset" UNQUOTE, except I read: if you are sharing a drive (partition) with Windows. OK, this I'm not doing, but I plan on "importing" all my emails into Ubuntu (Evolution or Thunderbird) ... and there are a "few" viruses out there for Linux (not "in the wild" ... what ever that means) So I figured clamav might not be such a bad idea ( there's that mindset again) and followed instructions I read here and got:

bruloo@The-Team:~$ sudo apt-get install clamav
Reading package lists... Done
Building dependency tree... Done
Package clamav is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package clamav has no installation candidate

  Do I really need it?
  I'm so new to this Linux stuff, I'll be born tomorrow!
  Now that's a newbie!
  Bruce Milmine