I don't think you need a virus checker with linux. Not operating as root is an automatic protection you don't get with windows. You should have a good firewall, though. I got cracked by a kiddie script before I got a good firewall which pretty well did me in as any user except root. You don't give your distribution, but if you have Red Hat, Seawall is a good ipchains-based firewall for Red Hat.
http://seawall.sourceforge.net
If you have Red Hat, you can use Pine mail system. While viewing the message, press 'h' to show all the headers. Reading from the bottom of the headers you can see the machine that the message originated on and compare it to the supposed sender. Good for the latest worm going around. Reading from the bottom, compare the last "From" name and IP to the last "Received" name and IP. If it's okay, they'll match. With some like yahoo, at least the IP servers will match.
To see if you've been cracked, install checkrootkit. You can put it in your cron jobs or run it as desired.
http://www.chkrootkit.org/