Binos,
You do not need any anti-virus software as there are no viruses for the mac. It's just not an issue. You will only start to use up resources on your machine from which you could benefit solely. I've been using macs for nineteen years and *never* had a virus. For the last ten years, I've been more-or-less permanently online. I don't even turn on the software firewall. Don't be dragged into the virus paranoia which affecs Windows machines so much. Mac OS X is a far superior operating system in so many ways, but especially in terms of security.
As far as I know, if the Mac software firewall is on and your PC is connected to the internet through it, the PC should be protected by it. You can specify which ports to open if necessary on the software firewall. However, once you get the broadband sorted, your router should have a hardware firewall built-in.
AppleMacster
(apologies if a little over-zealous above)