I didn’t realise that Snow Leopard includes a basic form of malware detection called File Quarantine. It’s a very simple virus checker, relying on on-demand, signature-based scanning. Terribly old fashioned stuff. Further to this, signatures were not updated other than in system updates. Practically useless.
Today’s security update from Apple addresses the up-to-date issue, a sure recognition that Apple recognises the security issues OS X is starting to face. Now File Quarantine will automatically update its signatures daily. This would have protected against MacDefender, but caution is still advised, as always. I suppose if these are your threats, quaintly old-fashioned defences will do for now.