SAN FRANCISCO – Intel’s sales are falling at a rate that blindsided the chip-maker’s management, amplifying Wall Street’s worries about the slumping personal computer market and the frail economy.

The foreboding news came out Friday in revisions to Intel Corp.’s financial guidance for its current quarter.

The world’s largest maker of computer chips now expects to post third-quarter revenue of $13.2 billion. That would represent a 7 percent decline from the same time last year when Intel’s revenue totaled $14.2 billion.

The projection also is well below a management forecast in July that envisioned third-quarter revenue ranging from $13.8 billion to $14.8 billion.

Investors punished Intel for the miscalculation, driving down the company’s shares by 90 cents, or 3.6 percent, to close at $24.19.

Intel blamed its sliding sales on lackluster demand for new PCs among businesses and a “challenging” economic environment.

Similar ailments are probably plaguing other companies whose fortunes are tied to PC manufacturing and sales. Intel chips are used in about 80 percent of PCs and a vast number of servers as well, making it a bellwether for spending on computers.

Taking their cue from Intel, investors bailed out a wide range of PC-related stocks. Companies whose shares fell Friday included Microsoft Corp., Hewlett-Packard Co., and other chip-makers, such as Advanced Micro Devices Inc. and Nvidia Corp.

The slowdown in PC sales will be the worst for the second half of a year in the industry’s history, Citigroup analyst Glen Yeung said.

The weakening PC sales in the corporate market are the latest sign of a technological shift driven by the growing popularity of smartphones and tablet computers.

Consumers have been buying fewer desktop and laptop computers as they embrace sleeker, more convenient mobile devices to surf the Web. That shift hasn’t been good for Intel because its chips aren’t used in a lot of cellphones and tablets.