My parents insisted I got an anti-virus program, even though even the Apple Store guy agreed with me that there were practically no viruses for OS X.
My advice is DON'T GET ONE. I have the Norton one for Mac, and it's annoying as ****. Whenever I insert a disc it scans it for not only the 0.5 known viruses for OS X, but also for all the 5 000 000 PC viruses out there. Whever I mount a .dmg file, it scans it. It's annoying as ****, and it only gets in the way.
Practically the only reason antivirus programs are made for Mac is so they can scan for PC viruses, so if you're in a PC network you don't infect everybody else. Seriously, there has never been a successful Mac virus, ever, in the history of mankind.
It's a waste of money, trust me. I would uninstall mine but my parents are a bit paranoid, like you. Well, I guess they have a reason, since they're not going to replace my PowerBook if anything goes wrong, since I'm only 13. But seriously, it ISN'T WORTH IT AT ALL.
Trust me.