All computer OS's are prone to viruses, I have been informed by Linux /
ubuntu users that Avast has a good antivirus for your needs.
This is true, but Linux is far less prone to viruses than windows just because of the way it works (and this is ignoring all the obscurity reasons why there's less viruses around for linux.) If windows was built the same way, we'd all have far fewer issues...
Thing is in windows, permissions are very lax. Everyone runs everything as an admin, all files / programs have read, write and execution permissions by default and they can do anything they want to the filing system as a result without any warning that they're doing it. Yes, before I get shot down I admit some of this has changed recently (with the likes of UAC introduced in Vista and more common limited accounts introduced in XP) but it's really just trying to put a sticking plaster on the grand canyon - most people still run as admin because a lot of programs don't work otherwise, and most people just click "allow" on every UAC box they see simply because it's what they've been told to do! As such, if you get sent a virus or if one is downloaded without your knowledge, it can run instantly and essentially do whatever it likes.
Let's compare this to the Linux world. If you download something through email, off the web or whatever then it WON'T be able to execute at all by default. Sure you can chmod +x it if you want it to, but you'd only do this if you knew it was safe - there's no way you're going to do that by accident. Even if this does run, everything in Linux is sandboxed so it won't have root permissions by default, sure it could trash your files but that's about all it could do. It can't install a million other programs, force itself to run at startup, muck anyone else's files up or embed itself deep in the filesystem like it could if it was running on windows, Linux just won't allow that to happen.
If you told any (half intelligent) Linux user to chmod 777 their entire filesystem and run everything as root they'd rightly tell you to shove your advice where the sun don't shine (or some derivative of the above words using stronger language!) Yet on the windows front, people do the equivalent of this daily!
This is one of the many reasons why Linux is far, far more secure than windows. It's not just because of obscurity and because users running Linux are generally more knowledgable - it's been built from the ground up as a far more resilient OS.
I haven't even started on things like SELinux which increase this protection further still...
As such, there's really no need for any virus / malware protection apart from in very special cases - most of the time it's just a waste of resources.