PC virusses will NOT effect your system in any way. The only danger you face is, being a carrier. You can transfer virusses to PC users. I run Virex on my 10.3.9 rig and nothing on my Tiger rig. I have only had one incident with a Cookie virus that did not effect my system at all, but....
If you are in a network with PCs it is wise to install some sort of antivirus cos you might be the only one that can stop it, but otherwise, it is of little concern.