Well one of the more well known AV app's is "ClamAV".
Don't forget, if you've only just arrived in the linux world, it takes a while to realise why MS have made the recent changes i.e. with the advent of XP.
Previous versions allowed executive permissions from the user account (which for pre XP was also the sys admin account).
At least with XP you can now just set up a sys admin account and seperate user accounts without exec capability - where have I heard of that before ????? Oh yes.............Linux
Provided you installed and set up the linux distro with a "stong" /root password i.e. upper and lower case, numbers and punctuation/special signs, then that's the first level of protection in place.
There are conflicting stories about how many virus' are actually "in the wild", but I understand it to be that many, that you can count them on the fingers of 1 hand.....
so no, linux isn't perfect. though it's fair to say that it's not so much virus' that are a threat, but though trying to gain access for other reasons, so maybe you should look into firewalling first. Lets face it, if "they" can't install malware, it can't be run, but you're correct to not be complacent.
Here at LQ is as good a place to start as any, or of course, googles linux servers
p.s. have you checked you installation disc's? I'd suspect that that's the best place to look for ClamAV or possibly something else (there was usually something on my discs when I ran mandrake).