A double dose of discouraging sales news shook the personal computer industry yesterday, leading analysts to conclude that the nation's deepening economic slump is punishing even one of its most robust businesses.

The most disheartening report came from InfoCorp, a California market research firm, which said U.S. sales of personal computers through independent dealers fell 25 percent in September from the monthly sales average of the previous eight months. This does not include direct sales to companies and schools.

Separately, International Business Machines Corp., while reporting overall increases in sales and profits, told analysts that personal computer sales in the third quarter fell short of its expectations.

Yesterday's disclosures are the latest confirmation that businesses are holding back purchases despite the potential labor savings and efficiency gains that computers promise.

"A PC depression may be just around the corner," analyst Rick Martin of Prudential-Bache Securities Inc. told the Dow Jones News Service.

"The economy every day looks worse and people cut back on spending, and there's a degree of market maturation. People who want to do a spreadsheet or want to do word-processing probably already own a PC," said Stephen Cohen, an analyst with SoundView Financial Group in Connecticut.

The prognosis for 1991 appears even gloomier: A recent SoundView survey of potential customers found that they planned to buy 25 percent fewer personal computers next year. And a sharp slowdown recently in European sales is likely to hurt PC firms that had depended heavily on Europe earlier this year to counter the weakening U.S. economy.

IBM, while not breaking out results for personal computers, indicated yesterday that sales didn't meet its expectations, analysts said. Overall, the company said profits in the third quarter jumped 27 percent, to $1.1 billion, on a 7 percent gain of revenue to $15.3 billion. But the seemingly impressive results disappointed Wall Street. IBM stock fell $1 to $99.25 yesterday.

Recession or not, the personal computer industry is pushing forward with what it does best -- introducing more powerful products for ever-lower prices. Yesterday, Compaq Computer Corp. unveiled what it says is the most powerful notebook-sized computer, which is expected to sell for around $5,000.

And Apple Computer Inc. launched three new machines, including a $999 model intended to attract price-conscious consumers. The company launched a $40 million promotional blitz in an effort to wipe out Apple's image as a high-priced brand. Asked recently how the firm prepared for launching new computers into the jaws of a recession, Apple Chairman John Sculley said the firm cut prices even lower than it had planned.

Indeed, price cuts of as much as 20 percent on some models have swept the PC industry in recent weeks, a trend that is likely to continue as manufacturers tempt consumers to put a computer under the Christmas tree. "It's going to be a buyer's market. We're going to see price erosion like we've never imagined," said JoeAnn Stahel, president of Storeboard/CI, a Dallas firm that surveys computer retailers.

Just how the year will end up is anybody's guess, but some analysts said things aren't as bad as they may seem. For one thing, IBM expressed hope that an upturn in sales in September will persist. For another, the new models and lower prices should at least sell more computers, even if companies don't earn as much per PC. So far, revenue has held up: InfoCorp's survey found dollar-sales in September were the same as a year earlier.

International Data Corp., a Boston area research firm, said it suspects that 4.5 percent more computers will be sold in the United States this year, about the same growth rate as registered last year.