MOUNTAIN VIEW, Calif. — With sales of computers deteriorating by the day, the PC industry’s dominant players — Microsoft and Intel — have arrived at the stark realization that the slump in sales could last a long time, perhaps years.

“We are certainly in the midst of a once-in-a-lifetime set of economic conditions,” Microsoft’s chief executive, Steven A. Ballmer, told investors Thursday in a conference call to discuss the company’s dismal second-quarter financial results. “Our model is not for a quick rebound. Our model is things go down, and then they reset. The economy shrinks.”

To help it cope with that lower base of demand, Microsoft said that it would lay off up to 5,000 employees, or about 5 percent of its work force — the first significant cuts in the company’s 34-year history. The layoffs follow a rare decline in sales of Microsoft’s Windows operating system for personal and business computers in the second fiscal quarter. Net income for the period, which ended Dec. 31, fell 11 percent to $4.17 billion.

Other major players in the personal computer industry are reporting similar drops in demand and paring their work forces to adjust.