well your actually receiving the internet straight from the modem and the wireless router not through the pc.
you dont need any virus protection for the mac as there are currently no Mac OS X viruses in the wild. if you do happen to download a windows virus or receive one through file sharing with your pc it will just sit there dormant and not do anything.
however there may be a chance that you can infect other pcs on your network. you can use clamxav to check this. however it is very unlikely.
just enjoy your mac and dont worry about these kinds of things!