I never had any problems running free ZA through a router.

Built in firewalls won't stop stuff from connecting from your competer. ZA would let you isolate the file so I could not connect to the net. If the free version doesnt work for, buy somthing. It is better than having to reinstall Windows.

I was able to run my system fine with a couple of worms and trojans I couldn't get rid of by isolating them with the firewall. That is until I turned it off and connected to the net one day. I lasted about 15sec.