"The Five Worst Security Mistakes End Users Make", SANS Institute, June 4, 2000:>

• Opening unsolicited e-mail attachments without verifying their source and checking their content first.
• Failing to install security patches-especially for Microsoft Office, Microsoft Internet Explorer, and Netscape.
• Installing screen savers or games from unknown sources.
• Not making and testing backups.
• Using a modem while connected through a local area network.
Using a modem on a network is exactly how cable modems in a neighborhood are hooked up. It also happens to be true of anyone hooked up to the Internet who is not connected from inside the firewall of an enterprise (company, agency, whatever). That means all home users. That is why you need a personal firewall.