What was going on when I was born?

The International Business Machines Corporation, apparently seeking to tighten its hold on the market for sophisticated desk-top computers, today introduced an advanced version of its personal computer that experts said was so powerful it could best be compared with far more expensive systems. The new Personal Computer AT is an office computer capable of performing numerous tasks at once. It can support the work of up to three people simultaneously, with two of them at remote terminals. Because the AT uses a new microprocessor, the Intel 80286, the computer works at two to three times the speed of older I.B.M. models. But its cost, $4,000 to $6,000, depending on the configuration of the equipment, is far lower than most dealers and experts had expected.

Sales of new American-made automobiles, held down by a shortage of cars, slipped eight-tenths of 1 percent in the Aug. 1-10 period from the same period last year, when sales were unusually strong. It was the first time this year that domestic sales have declined industrywide. Analysts have been expecting a decline in sales in the second half of 1984. L. Raymond Windecker, an analyst for the Ford Motor Company, said last week, ''Considering the low inventories and the recent above- trend sales that should require some payback, a flat period would be logical.''

An article in some editions yesterday, on restrictions for reporters covering the national Democratic candidates, misstated a rule change. A prohibition on reporting or photographing the activities of candidates during campaign flights without prior permission was suspended for journalists traveling with Walter F. Mondale but retained for those covering Representative Geraldine A. Ferraro. Later developments are reported today on Page B7.

After being cheered by the multitudes in the Los Angeles Coliseum, praised by the President in a personal audience and treated to ceremonial exaltation in our nation's capital, America's Olympic medalists are to be honored by the City of New York today with the traditional tossing of tons of trash on their heads from high buildings. ''Odd custom,'' said Richard Gaines, a tourist from London visiting Wall Street yesterday. Maybe so, but New York has so honored its conquering heroes since the first ticker tape was tossed out a window in the financial district and landed on Theodore Roosevelt during a parade honoring him in 1910. ''It's the least we can do to show our appreciation,'' said Roseanna Nuciforo, an employee of E. F. Hutton, who was collecting all manner of scrap paper yesterday to throw out the window at the 221 Olympic stars expected to march in a ticker-tape parade at noon today up Broadway from Battery Park to City Hall.

LOIS SEWELL had braces put on her teeth the same day her first grandchild was born. She was 50 years old then. Nine years later Mrs. Sewell, who is president of an electrical supply company in a Boston suburb, says she doesn't regret a minute of her year-long orthodontic treatment. Roland Rattee is another of the more than 850,000 adults who wear braces. ''Before I got braces my mouth looked prehistoric,'' the 34-year-old Manhattan accountant commented. Five years after beginning the treatment he is amazed by the improvement in his appearance. Recent developments in orthodontics as well as a pervasive emphasis on beauty and health have made braces more acceptable among adults. Many of those who wear them have lived with orthodontic problems for years, but because of a lack of knowledge or money - the cost usually begins at about $2,000, with dental insurance sometimes covering a part or all - have not taken corrective measures. Some, like Mrs. Sewell, need orthodontic treatment as a prelude to bridgework or to help stop gum recession or bone loss. ''My dentist never told me'' is a frequent reaction among adults when they finally see an orthodontist.

Governor Cuomo announced today that he had signed the first law in the nation specifically intended to curb the pollution known as acid rain. Environmentalists hailed the legislation, saying they hoped it would prod other states and, ultimately, Congress and the President to act against acid rain. But business groups, who had urged Mr. Cuomo to veto the measure, said its costs could be high and its benefits uncertain. The new law would require reductions in the emission of sulfur dioxide, believed to be one of the prime contributors to acid rain.

The Europeans were surprised at Harlem. Surprised, perhaps a little bewildered - and clearly delighted. They had no idea that this was ''Harlem Week'' and would be until next Sunday; that lots of people were being encouraged to visit Harlem's restaurants, museums and churches; that Citibank had just made a $65,000 grant to get the newly formed Harlem Tourism Association going, and that a growing number of people in Harlem had reached the conclusion that the history and culture of the place were such that it ought to become more of a tourist attraction. As far as the Europeans were concerned, Harlem was quite a tourist attraction already, even though the tourism association has just been created.

With United States minesweeping forces headed for Red Sea shipping lanes off Egypt and Saudi Arabia, there is still no evidence to determine who planted explosives in the waters, Administration officials said today. Michael I. Burch, the chief Pentagon spokesman, said the Shreveport, an amphibious transport dock loaded with four RH-53D minesweeper helicopters and 200 servicemen, was in the Mediterranean today and would arrive in the Gulf of Suez later this week. The airlift of three helicopters and about 120 men from Norfolk, Va., directly to Jidda, Saudi Arabia, was completed Monday night, he added. Mr. Burch said Egyptian forces and American personnel on board the Harkness, a Navy oceanographic research vessel, have had no success in discovering the cause of 17 explosions reported by commercial vessels in the Red Sea in July and August. He said efforts were hampered last weekend by rough seas and high winds.

The New York Stock Exchange, as part of its new effort to strengthen its competitive ability, said last night that it would like to trade a new class of shares of the General Motors Corporation. It also dismissed the possibility that it might no longer allow G.M. stock to trade on the exchange. The exchange statement followed the announcement late yesterday that General Motors and Electronic Data Systems Inc. had reached a definitive agreement for G.M. to acquire the Dallas-based data-processing company for about $2.5 billion.

Broad changes in New Jersey's insurance laws that were intended to allow motorists to reduce their premiums have failed to gain wide acceptance, according to state officials. The new law, enacted late last year, was expected to give New Jersey drivers relief from what had become the highest average insurance premiums in the country. The law allows motorists to reduce their premiums by as much as 25 percent by buying less protection than they were required to have under the old law. But officials monitoring the acceptance of the new insurance options over the last nine months now believe that it may take years before a majority of the motorists take advantage of them.

''Odd custom.'' - Richard Gaines, tourist from London, on ticker-tape parades. (A1:2.)

be it Hunan or Sichuan, Tex-Mex, Thai, Indian or Louisiana Cajun - has led to a surge of interest in cooking with hot peppers. Many people who first became enamored with hot foods in restaurants are now running out to ethnic or specialty stores and loading up with all sorts of peppers so they can try their hand at home. It doesn't take long to realize, especially if you are on the receiving end of these experiments, that peppers in the hands of an inexperienced cook can be dangerous. There are a few fundamental rules that apply to cooking with hot peppers. One of the foremost practitioners of this art, Paul Prudhomme, owner of K-Paul's Louisiana Kitchen in New Orleans, refers to an ''after-swallow glow'' that is the hallmark of a well- prepared peppery dish. It can be achieved, he explained, by understanding the specific properties of different spices and how they interact with other foods. ''The secret,'' Mr. Prudhomme explained in a tele