Stocks slumped as fears about the cooling economy's impact on companies' growth became more anguished with Microsoft warning investors to expect less profits and revenues.

Trading volume was very heavy, and Friday's "triple witching" expiration of stock options, index options and futures exaggerated the market's swings.

For the week, the Dow dropped 2.6 per cent, the Nasdaq was off 9 per cent and the S&P 500 fell 4.2 per cent. So far this year, the Dow is down 9.2 per cent, the Nasdaq 34.8 per cent and the S&P 500, 10.7 per cent.

Microsoft lost nearly 12 per cent, or $65/16, at $49-3/16 after tumbling to $47-3/4, its lowest level in more than two years. Nearly 158 million shares of Microsoft changed hands.