Terry Sanders didn't notice that something was different about her home computer until she was about to shut down after an online session.

There on her desktop was a new icon with the ominous name "I was here." Sanders determined that it was a text file and, thinking there'd be no harm, opened it. The message inside addressed her with a chipper hello and asked how the weather was in St. Petersburg, Florida—where she lives.

The note went on to inform her that the writer had been on her PC, and unless she did something about her security problem, he'd be back to do some damage.

Since Sanders is a senior design specialist for a firm that makes human resources and payroll software, she might be expected to know something about security. But she admits that the topic made her head swim. Until that day, all she knew about firewalls was that they were a nuisance when she was trying to dial into the office from home. She knew that her cable modem made her vulnerable to intruders but had never given serious thought to security. She assumed that the cable Internet provider would have software in place to keep her safe.

After her strange attack, she asked coworkers for advice. Each recommended a different solution. She ran antivirus software and found that both of her home PCs were infected. Luckily, the only damage was some trashed text files. She cleaned both computers, installed a hardware router with a firewall, and hasn't had any more break-ins.