Shares of Acer stock were down nearly 7% on the Taiwan Stock Exchange Tuesday on news that the PC manufacturer had unexpectedly and significantly lowered it’s first-quarter sales estimates. The PC maker had previously estimated its first-quarter PC sales would grow 3% from the previous quarter. Now, Acer has adjusted its forecast, saying it expects sales to drop 10% compared to the prior quarter. Asustek, an Acer competitor, is also struggling with similar sales declines in the first quarter. Acer cites weakened demand in the U.S. and Western Europe for the drop off in sales. The emerging tablet market lead by Apple’s iPad and iPad 2 devices is also seen as largely contributing to the decline in PC sales. Acer expects its net profit margin to drop below 2% in the first quarter from 2.93% in the previous quarter, though it predicts that second-quarter PC shipments will reverse the downward trend, growing by 10% over first-quarter sales.