What evil haunts your computer? Internet users beware: Zombies are among us.

Timing their effort to coincide with national Cyber Security Awareness Month and Halloween, the U.S. Federal Trade Commission (FTC), Consumer Action and Microsoft Corp. are urging consumers to protect themselves from the threat of “zombies,” computers that are infected with malicious code so they can be controlled remotely by other people for illegal purposes. Through technological trickery, criminals can use these unconscious accomplices to send illegal spam, launch phishing campaigns to steal personal information, attack Web sites and computers, or engage in other illegal activity.

Unlike the zombies of B-movie imagination, which are easily identifiable by their typically gruesome appearance and menacing groans, zombie computers are silent stalkers. People who use the Internet may never know that their computers have been compromised and turned into a conduit for sending millions of pieces of illegal spam or facilitating other illegal activity. More than half of all spam is sent through infected computers, according to industry reports. (A graphical explanation of how zombies operate is available.)

To combat the zombie threat, Microsoft today revealed some of the technological and legal maneuvers it has used to unmask the individuals using several zombies to send spam. Microsoft investigators intentionally created a zombie computer, quarantined it to prevent it from actually sending spam messages, then carefully watched it for 20 days while investigators tracked and traced all Internet communications through the infected computer.