Linux

try SOPHOS AV

It will install differently on different Linux distros, or depending on the installer used. What distro are you using?

If NONE currently, there are a few really good learner ones. Personally I prefer PCLinuxOS as a small-fast distro, and SUSE 10.2 as a more bloated, but fuller distro.

Also I would recommend PC BSD for a starter, it is not Linux, but it is very similar. Ease of installs is downloading and accepting the license, saying OK to install.
viola, done.
easier than Windows in my opinion.

http://www.clamav.net/

It's pretty easy to install and you can get it via most package managers.

let the windows users cover their own butts, why install antivirus when it only stops windows viruses.

best antivirus tool is anying NOT windows based.[ ie, linux, freebsd, openbsd, netbsd, irix, hpux, aix, solaris, pcbsd, desktopbsd .. ]
no virus infestations then.
[ and openbsd is running at "Only two remote holes in the default install, in more than 10 years!" ]