I'm helping out at a friend's office with about 15 PCs. Most are badly infected with spyware and viruses. There isn't a wireless connection so the only way I can connect my macbook is to directly plug into his network. How vulnerable am I to infections? Is the Mac truly immune to virus/spyware attacks? I've read that the Leopard firewall is pretty bad. Is there a third party app that might give me the security I need?