A Trojan horse, aptly named after the seemingly harmless tool of destruction in Homer's Iliad, secretly carries often-damaging software in a "plain wrapper." The plain wrapper is normally an e-mail file attachment from someone you may or may not know.

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A virus can cause your PC to act in strange ways -- from going totally dark to not launching programs to mysteriously misplacing files.

A computer virus is just what it sounds like: A program that alters the way your computer operates, affects its overall wellbeing, and spreads like wildfire among computers that "come into contact" with each other via the Internet.

Nonetheless, they have been very effective at shutting down major e-mail systems.

See How Computer Viruses Work for details on viruses.

Users are strongly advised to run up-to-date desktop virus scanners.

Virus scanned messages are rejected when: Rejected messages do return an error code to the connecting mail relay (which may be a desktop email client, or a central mail server) explaining why the message was rejected. Response: Messages such as these are the result of someone's computer that is infected with a virus sending an email message.All email sent to edu (incoming mail to you as well as outgoing mail from you) is scanned for known viruses.Note, however, that email is not the only way viruses spread.Never mind the embarrassment and sheepishness you feel when it looks like you’re spamming all your friends.What’s worse is that your online security and perhaps your computer have been compromised.Microsoft Outlook (or Express) with Internet Explorer 5 must be used in order for the worm to propagate.