They come with varied subject heads. CONFIDENTIAL AND TOP SECRET. READ CAREFULLY (urgent reply needed). ASSIST ME PLEASE. I WANT TO INVEST ABROAD. A CRY FOR HELP.

And the messengers vary: Peter Obi, eldest son of the late Henri Obi Kabongo. Eduado De Mello, Principal Commander of the Union for the Total Independence of Angola. Aaron Hassan, accountant for the Nigerian National Petroleum Corporation. Femi Martins, executive director with the Zenith Bank of Lagos.

But they all have certain things in common. The grammar-challenged English text. The plea for assistance in moving large sums of money from their country to yours. And the promises of riches to those who help in the transaction.

Who among us at Berkeley Lab has not received one of these emails? Or dozens? If so, you are among the millions who have been randomly targeted by what has become known over the last 10 years as the "Nigerian Scam." And these unsolicited offers - an illegal variation of electronic "spam" - keep on coming, because they work. The Secret Service, which has been tracking this con game for years, estimates that it grosses hundreds of millions of dollars annually for the cybercrooks preying on the world's unsuspecting public.

"This week alone, I got 40 spam messages," says Schultz, a computer systems engineer, of those unwanted solicitations, whose nickname derives from a Monty Python comedy routine. "It's cheap for them to do, and they need to get only a few responses to have a certain percentage of success."

In most cases, Schultz says, spam's impact is mainly to annoy the receiver. But in some cases, when the intent is to bilk someone or disrupt operations, consequences are more serious. He distinguishes between the hoaxsters, the hucksters and the scammers. All use spam, but for different purposes - to cause mischief, to sell something, or to commit fraud.

So, what to do? Schultz says the Lab soon plans to use a new, more sophisticated spam filtering software, called "BrightMail." But it won't eliminate all spam on your computer. So he has some tips to maintain some level of control.

1. Don't forward the spam to someone else, thus extending its reach. There's one exception - if it's a repetitive pitch, send it to spam@lbl.gov with the subject that reads "spam:block:" followed by the spammer's address. The source address will be added to the Spam Wall filtering system. In all other cases, just delete.
   
2. Don't open attachments from spam. That's how viruses and worms infect systems.
   
3. If you're a Web surfer, don't use your Lab account; set up a second home account for such searches. Web sites sell their address lists, and spammers cull them by searching the "@" symbols. Think twice, too, before leaving your email address at a web site.
   Don't delete files from your operating system, no matter what a spam message tells you (Schultz says that this is one of the most effective ones he's seen. A writer sends an email, apologizing to the recipient for infecting his system and providing a fix via a deleted file. The move could disable computer functions or damage enabling software).
   

Silly? The Secret Service says it receives about 100 phone calls, plus 300 to 500 pieces of related correspondence, per day from victims of the Nigerian scam. The average reported loss is $5,957, which was solicited by the scammers to ostensibly cover taxes, advance fees and bribes.