there is av software available for tiger...like norton 10. currently there isn't viruses for the mac...the thing that you now would be concerned about is rootkits and trojans. but the basic mac firewall should prevent most of these, just as long as you don't download software from untrusted sources (p2p, hacker sites, ect). I've found out there are some vulnerabilites in mac osx where some trojans could be installed, so having an av or an anti-trojan software can be beneficial in the long run.
i also recommend using a nat router as a firewall, and a different software firewall that blocks outbound traffic...but then again, i'm paranoid 🙄