What anti malware is ok to use for my mac?

I think my email contacts list has been "harvested". Since Sat. evening (nov. 5,2011) many of my contacts have sent me emails asking why I have sent them advertising for "Canadian Healthcare". I have replied that I haven't sent anything in several days and surely not ads for healthcare.

I have changed my email account password but some contacts are still receiving the ads. I'm afraid that my contact list is no longer under my control and wonder if there could be some malware program still hidden/hiding on my machine. I have run Symantec antivirus but dont know if that is enough. I have read that not all malware can be fully removed by antivirus software and was wondering which anti malware is best for macs?

Any suggestions/recommendations?

(I have two macs, an iBook g4, a macbook pro core duo model a1150, and an acer travelmate which is seldom used since acquiring the macs)