Jim is correct. Your computer is connected to the Internet, so it is on a network. Since you are connected to your home router, which is where your other PC connects wirelessly to, as well, you are actually on a Home network. OneCare's firewall needs to know this from you. If you select "Home or Work" for this setting, OneCare relaxes some firewall functions to allow file and printer sharing within your local LAN, but it does not create a network. You've already done that by plugging into a router. If you decide to select Public, file and printer sharing will be blocked and the connection to your PC is considerably more restrictive. The correct setting in your case would be Home or Work.
-steve