Helpful Links Regarding Flashback Trojan and Virus Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read the FAQ on malware.
For general anti-virus protection I only recommend using ClamXav.
Tom Reed has tested several AV utilities including ClamXAV only to find that none of them are particularly reliable. Fortunately, there are no viruses in the wild to worry about. Tom's Malware Guide is a must read.