Follow these tips to keep your data safe when using wireless networks:
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If you use WEP encryption, change your encryption key once a month. If someone manages to learn your key, they will be locked out again when you change it.
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Don't connect to unprotected wireless networks?it's possible for someone to monitor your Internet usage and even record your passwords.
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If you do connect to an unprotected wireless network, don't visit a Web site that requires a password unless the Web site is encrypted. To find out if it's encrypted, look for a lock symbol in the lower-right corner of your browser.

http://www.microsoft.com/windowsxp/using/networking/security/wireless.mspx

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Top 10 Tips for Wireless Home Network Security

http://compnetworking.about.com/od/wirelesssecurity/tp/wifisecurity.htm