How malaware spreads on your computer?

Finally, your network connection (both to your LAN and to the Internet) can serve as a conduit for a worm, the special kind of virus that doesn’t need your help to infect your system. Obviously, the most effective way to protect your system is to unplug it from the network, but a slightly more realistic solution is to use a firewall. Vista comes with a built-in firewall,
although a router provides much better protection.