U.S. stocks tumbled Monday, with major indexes falling by their biggest margin in months, amid concern about Google (GOOG) Inc.'s valuation and doubts about holiday spending after a disappointing sales estimate from Wal-Mart Stores (WMT) Inc.

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The Dow Jones industrial average was down 158.38 points, or 1.29 percent, at 12,121.79. The Standard & Poor's 500 Index was down 19.02 points, or 1.36 percent, at 1,381.93. The Nasdaq Composite Index was down 54.34 points, or 2.21 percent, at 2,405.92.

For the Nasdaq, it was the biggest net point decline since Sept. 24, 2003. The S&P 500 notched its biggest percentage decline since early June and the Dow fell its most since early July.

In addition, downward pressure on the dollar for a fourth straight day hurt demand for U.S. investments, while a rise in crude oil prices above $60 a barrel added to concerns about consumer spending.

While stores were busy on Black Friday, with more retailers opening their doors at midnight on Thanksgiving night or even earlier to capture eager shoppers, analysts worry that the high traffic may not lead to fat margins. Wal-Mart started its heavy discounting weeks before Thanksgiving, while other retailers beefed up their promotions just ahead of Black Friday.

"The dollar weakness has caught some by surprise, but what's mostly going on is we're getting the first blush of what we might see for Christmas," said Paul Nolte, director of investments at Hinsdale Associates, in Hinsdale Illinois. "Wal-Mart has been indicating some weakness for some time, and they are a bellwether for the retailers. It might not just be a Wal-Mart issue".

Ford Motor Co. (F) shares fell 4.2 percent, or 36 cents, to $8.16 on the New York Stock Exchange after it announced plans to borrow $18 billion by pledging assets as collateral to fund its restructuring.

In another sign of investor worries, the Chicago Board Options Exchange Volatility Index jumped 14.6 percent Monday. The indicator, also known as the VIX and the market's fear gauge, measures expectations of near-term volatility determined by the Standard & Poor's 500 options prices.

Shares of Wal-Mart, the world's largest retailer, fell 2.7 percent, or $1.29, to $46.61 on the NYSE after it estimated November sales fell 0.1 percent at U.S. stores open at least a year.

Customer foot traffic in stores over the long holiday weekend slipped from last year, the National Retail Federation reported, adding to worries about retailers' fourth-quarter performance. The S&P Retail Index was off 1.1 percent.

Shares of Web search company Google (GOOG) fell 4 percent, or $20.25, to $484.75 and were the biggest drag on the Nasdaq after the report in Barron's.

The dollar's recent decline to a 20-month low against the euro of $1.3172 has raised questions about the attractiveness of U.S. stocks to foreigners.

U.S. crude oil for January delivery climbed $1.08 to settle at $60.32 a barrel. Saudi Arabia's oil minister said OPEC may cut output further at a Dec. 14 meeting.

Shares of U.S. airlines fell on Monday, with Continental Airlines (CAL) down 7.3 percent, or $3.31, at $42.07 in Big Board trading, partly on rising oil prices. Shares of AMR Corp. (AMR), the operator of American Airlines, slid 5.6 percent, or $1.90, to $32.20 on NYSE.

Trading was active on the NYSE, with about 1.61 billion shares changing hands, matching last year's daily average of 1.61 billion, while on Nasdaq, about 2.00 billion shares traded, above last year's daily average of 1.80 billion.

Advancing stocks outnumbered declining ones by a ratio of more than 4 to 1 on both the NYSE and the Nasdaq.

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