'Linux may not be as vulnerable as Windows, but if you think Linux viruses don't exist, you'd better think again. Virus writers have any number of possibilities'

I have just read this sentence and I'm concerned because I have only firewall(from router a from FC4) working on FC4. Could you explain to me wich actions I should take? Note: I have Toshiba laptop, FC4, Gnome and Thunderbird. The only programs I know are Clamav and Spamassassin. Is it enough? Although I know FC4 has SELinux. Best regards, Joao.