Dini S, Brooklyn, New York

Short answer: Not very. You should never, for instance, sent a credit card number over the Internet via a café's or library's Wi-Fi connection.

Long answer: If a network is open to anyone, it's open to criminals. In fact, it's possible that you're not even connected to the café's server, but to an evil twin--someone else's computer that's acting like a server to gain access to your PC. Luckily, there are precautions you can take.

Don't go online without knowing it. Shut off your Wi-Fi if you don't need the Internet. That will save batter power, as well.

Make sure you're using the right network. When you log onto a wireless network, Windows will show you the Service Set Identifiers (SSIDs) of all the networks within range. Make sure you're getting onto the right one (you may have to ask an employee).

Turn off file and printer sharing. In XP:

1. Select Start and right-click My Network Places
   
2. In the resulting Network Connections window, right-click the network in question and select Properties
   
3. In the resulting dialog box's General tab, uncheck Client for Microsoft Networks and File and Printer Sharing for Microsoft Networks. Click OK.

Vista does this automatically if it recognizes the network's unsecured status--which it always has in my experience. To check and possibly change this setting, do the following:

1. Select Start, Network
   
2. Click Network and Sharing Center
   
3. If it says "(Public network)" next to the network name, and you're actually connected to a public network, simply close this window because you're done. If it doesn't:
   
4. Click the Customize button across from the network name
   
5. Select Public, click Next, then Close. Be careful what you do. Never make a purchase, use online banking, or enter anything sensitive from a public Internet connection. Avoid using passwords as much as possible.

Be careful what you do. Never make a purchase, use online banking, or enter anything sensitive from a public Internet connection. Avoid using passwords as much as possible.

Go to this list of information you should be wary of sharing online. Of course that list is meant for a private Internet connection. On a public network, the word isn't wary, it's never.