NEW YORK (CNN/Money) - U.S. stock investors have their work cut out for them Thursday, as they sort through earnings reports from General Motors before the open, and IBM, Microsoft and Sun Microsystems after the close, not to mention Yahoo!'s results, released late Wednesday.

Late Tuesday, Intel (INTC: down $0.44 to $17.35, Research, Estimates) became the first of a number of high-profile technology companies to report results this week. The firm's numbers beat estimates, but its conservative forecast sent markets tumbling Wednesday.

After the close of trade Wednesday, Yahoo! (YHOO: down $0.12 to $19.58, Research, Estimates) reported a fourth-quarter profit of 8 cents a share, 2 cents better than expected and 6 cents better than a year earlier. But shares of the stock fell more than 4 percent in after-hours trading on Island ECN, perhaps due to the Internet company's ho-hum forecast.

Yahoo! is forecasting first-quarter revenue in a range between $255 million and $275 million, when analysts surveyed by Briefing.com expect revenue of $270 million. The company also forecast revenue of between $1.1 billion and $1.2 billion for the full year 2003, when analysts are expecting revenue of $1.2 billion.

Additionally, PC-maker Apple Computer (AAPL: down $0.18 to $14.43, Research, Estimates) said after the bell that it earned 3 cents per share in its fiscal first quarter, in line with estimates and down from the 11 cents it earned a year earlier. Looking forward, the company forecast second-quarter revenue of $1.47 billion, essentially in line with current estimates of $1.4 billion. Shares of the stock dipped 20 cents to $14.23 in after-hours trade.

However, the Yahoo!-Apple impact on Thursday's market could be muted, as "neither is a bellwether," Erik Gustafson, senior portfolio manager at Columbia Management Group, told CNNfn's Street Sweep. "The bellwether is tomorrow, when IBM reports."

Both IBM and Microsoft are due to report results after the closing bell Thursday.

But even before investors take in those tech leaders, General Motors (GM: down $0.30 to $40.20, Research, Estimates) is due to report its fourth-quarter results before the opening of trade Thursday. The automaker is expected to have earned $1.53 per share, according to analysts surveyed by Briefing.com. This would mark a huge improvement over the 65 cents it earned a year earlier.

After the close, IBM (IBM: down $0.99 to $87.59, Research, Estimates) is expected to report a profit of $1.30 a share, down from the $1.46 earned a year earlier. Additionally, late Thursday, Microsoft (MSFT: down $0.70 to $56.27, Research, Estimates) is expected to post a profit of 46 cents for its fiscal second quarter, down from 49 cents a year earlier. Microsoft and IBM, both Dow stocks, fell Wednesday.

"I think that the IBM numbers in particular are going to be important," said Brett Gallagher, head of U.S. equities at Julius Baer. "The chatter that I hear is that Microsoft should beat, but IBM is more mixed. So if IBM beats, that might drive the market higher."

Sun Microsystems (SUNW: down $0.03 to $3.87, Research, Estimates) also reports after the bell Thursday and is forecast to have lost 2 cents a share in its fiscal second quarter; it lost 3 cents in the year-earlier period. The company's stock lost 1 percent Wednesday afternoon.

eBay (EBAY: down $0.68 to $73.50, Research, Estimates) is expected to report earnings late Thursday of 24 cents a share for its fiscal first quarter, up from 14 cents a year earlier. Shares slipped ahead of the report.

Thursday also brings economic reports on consumer prices, the Philadelphia Fed index and weekly jobless claims.

"We've seen more bad news than good news on the economic front over the last few weeks, but the market has been trading higher," Julius Baer's Gallagher added. "Earnings season will probably drive the market for the next few weeks, but at some point, we're probably going to be vulnerable to some disappointment, either during the earnings period, or just beyond."

The Dow Jones industrial average (down 119.44 to 8723.18, Charts), the Nasdaq composite (down 22.19 to 1438.80, Charts) and the S&P 500 index (down 13.44 to 918.22, Charts) all lost more than 1 percent, a slight improvement from their worst levels of the session.

Investors played it fairly safe in the early part of the week, showing a reluctance to make any moves ahead of the release of profit reports from the tech heavyweights. But this changed with the release of Intel's fourth-quarter earnings report.

Intel's results topped analysts' estimates, but the company's outlook on the near future was pretty conservative. That led to selling in the stock Wednesday and general caution about the prospects for a recovery in the technology sector.

Companies that make the equipment Intel and its competitors use to make chips suffered. Among the damaged stocks, Applied Materials (AMAT: down $0.91 to $14.45, Research, Estimates) lost more than 6 percent and KLA-Tencor (KLAC: down $1.20 to $38.45, Research, Estimates) lost nearly 3 percent. The Philadelphia Semiconductor index, or Soxx, lost more than 3 percent.

The blue chip index, of which Intel is a member, was also knocked lower, with 29 of the 30 stocks that comprise the Dow industrials losing ground. AT&T (T: up $0.19 to $27.46, Research, Estimates) was the only gainer.

In other corporate news Wednesday, shares of Dow component DuPont (DD: down $1.50 to $42.50, Research, Estimates) fell almost 3 percent after the second-largest U.S. chemical maker warned that fourth-quarter results could miss current estimates due to a weaker-than-expected second half of the quarter. DuPont expects earnings per share of 31 to 33 cents, while analysts were looking for 33 cents.

In addition, shares of aluminum producer Alcoa (AA: down $0.96 to $22.20, Research, Estimates) fell, losing more than 5 percent, pressuring the Dow industrials.

"The mining sector is down today and it's dragging Alcoa with it. As the market weakens, this kind of cyclical stock is often punished more than stocks in other sectors," said Daniel Brebner, a mining analyst with UBS Warburg who covers Alcoa.

"Also, the company missed estimates when it reported results last week and announced some restructuring moves, including layoffs, and people are still reacting to that," Brebner added. "I'm looking for more weakness in the stock near-term."

Market breadth was negative. On the New York Stock Exchange, decliners beat advancers by 5 to 3 as 1.37 billion shares traded. On the Nasdaq, losers beat winners by 10 to 7 as some 1.65 billion shares changed hands.

Bonds were little changed, with the benchmark 10-year note rising 1/16 of a point in price, yielding 4.07 percent. The dollar was a little weaker against the euro and the yen.

In commodities markets, light crude oil rose 67 cents to $32.45 a barrel in New York, hovering near two-year highs. Gold dropped $1.30 to $351.10 an ounce in New York.