The program in Windows XP used for finding wireless Internet access shows two kinds of connections -- access points, which let you jack in to the Internet, and ad hoc networks, which provide access to a standalone computer.

The Windows icons for these two are different. An access point is designated by an antenna; an ad hoc network looks like two notebook computers side by side.

Generally, you want to avoid ad hoc networks in public places. At best, they're a radio signal to nowhere; at worst, they could be someone looking for a vulnerable PC to compromise.

The "Free Public WiFi" connections I was seeing were all ad hoc networks. Just out of curiosity, I connected to one once. Nothing happened.

When Windows connects to a network, it retains that network's name, or SSID, then broadcasts its as an ad hoc network, essentially inviting a connection. You can find more details here. Microsoft has said it will fix this in the next XP service pack; it's unclear if Windows Vista behaves this way.