"You may already be infected!" So says an e-mail from your friend or coworker, instructing you to find and delete an obscure-sounding file (Jdbgmgr.exe) buried in your Windows directories and then forward the message to everyone in your address book. Of course, you aren't infected; the file is a Java component, and the e-mail is a hoax.

Internet-borne viruses are devastating, but virus hoaxes can be devastating as well. "Some of these hoaxes have cost companies thousands of hours in downtime," said Chris Wraight, technology consultant at Sophos, an antivirus software company (www.sophos.com). "I encourage companies to have a continuity plan to respond to hoaxes, just as they respond to a virus or security breach."

Sophos's site includes virus hoaxes, chain letters, and scams in its hoax category. It estimates that there are thousands in circulation.

When you receive an e-mail message that may be a hoax, Wraight suggests checking with a reputable expert before doing anything else. "Send the letter to your IT department, not to friends," he says, "or check with a security site to see whether it's a hoax or legitimate."

Sophos maintains an updated list of viruses and hoaxes, as do antivirus sites such as www.symantec.com and scam-busting sites such as www.snopes.com/computer/virus.