AVG and ZoneAlarm are for PC only: if you want anti-virus for a Mac, then you need either Symantec or Sophos.
Symantec also make a Firewall but OS X has it's own built-in. See click here
It's unlikely anyone will bother to try and hack into it or write viruses for it because only web designers and a handful of architects ever use Macs.
"Our first experience of using a Mac.." yeh, and probably your last: Macs are pointless pieces of overstylised California, free-love, hippie-s**t, allegedly...
Hope that helps.