Mac OS X has got its own security systems, so you don't need an antivirus. See > http://www.thesafemac.com/mmg
If you want an antivirus, you can use Sophos or ClamXav, but you don't need it. You just have to keep the Mac updated to the most recent version
Helpful Links Regarding Malware Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
If you require anti-virus protection I recommend using ClamXav.
Browse through the "More Like This" section over to the right... You'll see there are plenty of discussions on this topic.
Most people, myself included, would say, "None. You don't need an antivirus program on a Mac."
I have owned Macs since 1999 and have never once installed an antivirus program on a single on of them.
Keeping your system software up to date will help prevent viruses and malware.
You can check for updates anytime. Click Software Update from your Apple menu
Your Mac can check for updates for you.
Open System Preferences > Softwate Update
Check the following:
Automatically check for updates
Download newly available updates in the background
And you can click Check Now anytime.