It's a two phase process. Get a router. Whether you have one or several PCs, the router has a firewall. Next, get ZoneAlarm free firewall.

The hardware firewall in the router is the first defence, the software firewall is the second defence. And ZoneAlarm will block outgoing attempts.

I have a D-link setup - cable modem, router, three wired PCs and one wireless laptop. All running ZoneAlarm. I have had zero intrusions.

Good luck,
Telephony facts, analog/digital explained.
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I think computer viruses should count as life. I think it says something about human nature that the only form of life we have created so far is purely destructive. We've created life in our own image. - - Stephen W. Hawking