"Congratulations!" reads the envelope. "You have won a free weekend at Seventh Heaven Lakes."

"If you'll just take a few minutes to participate in our survey," says the voice on the telephone, "we'll send you a valuable prize."In the era of junk mail and junk calls, most Americans are familiar with well-disguised pitches for products and services of dubious merit. They have developed a healthy sales resistance.

Yet when it comes to investing their money, many people still seem to be vulnerable to hucksters and con artists who play on their greed.

"Americans lose hundreds of millions of dollars every year to investment fraud," say the Council of Better Business Bureaus and the North American Securities Administrators Association in their recently published book "Investor Alert!"

"It touches all segments of society - the rich and the poor, the young and the old."

One thriving, and especially virulent, institution is the "boiler room," from which a squad of salesmen push real estate, gold, commodity options or some other deal by telephone.

"Boiler room operations are thriving because investors, including some normally sensible ones, persist in falling for deals that come in cold by telephone," says Brad Hitchings, an analyst at Standard & Poor's Corp., in S&P's advisory publication The Outlook.

"The pitch is often highly convincing - always plausible and well-laced with correct technical jargon."

And as the BBB and NASAA observe in their book, "The lures always include assurances that you can take the next step with absolutely no commitments, see results quickly and get your money back at any time if you are not satisfied with the deal."

Boiler-room operators can make false promises in abundance because they are adept at disappearing quickly, only to appear again with a new scam under a new name at a different address.

How can you protect yourself from financial predators like these? And how you can distinguish between a con artist and somebody with a legitimate proposition to offer that might actually pay off?

Experts on the subject say it is wise to assume the worst when you get an unexpected call (or letter or visit) from some stranger whose firm you have never heard of.

"When in doubt," the BBB and NASAA advise, "make no promises or commitments, no matter how tentative. It is far better to wait and lose an opportunity than to take the plunge and lose everything.

"When hounded on the phone by a promoter, don't be afraid to hang up without explanation. You do not owe the caller anything - in fact, this kind of solicitation is an invasion of your privacy."

If you don't adopt a "just say no" policy, advisers say, do a lot of independent checking before you make the slightest commitment to any deal.

When a promoter insists an opportunity will be gone by the time you make your own inquiries, it is almost surely a sign the proposition is questionable or a downright fraud.

"Investor Alert" tells the story of a widow who was encouraged to invest in silver because International Business Machines Corp. was supposedly developing a new breed of computer that required pure silver circuitry, which would cause the price of the metal to soar.

She followed the precept "investigate before you invest," and determined that the stories of new solid-silver technology had no basis in fact.

"The silver dealer turned out to be an old hand at gold and silver swindles," the book reports, "and was already folding up shop and moving to a new location and a new con game."