there were a few linux viruses in the past, but the security holes they exploited were fixed... at the present time there are no known linux viruses...
if a (new) linux virus is released, YOU'LL KNOW - cuz it'll be all over the news...
anyways, if you are sharing files with windows machines, an antivirus on your linux box is a good idea, as it would suck to unknowingly send your friends/family/coworkers a windows virus that was harmless on your linux box but infects their windows system...
there's a free (as in freedom) virus scanner here:
http://www.clamav.net