Apple Mac unit sales were down an estimated 6% year-on-year last month, well below Wall Street estimates of a 4% fall.
If there is a year-on-year quarterly fall in Mac sales, it would be the first such dip since 2003.
The sales dip for January is estimated by market research firm NPD, which also says iPod sales fell 14% year-on-year.
Financial analyst firm Piper Jaffray, which is heavily pro-Apple in the financial markets, said Apple's second quarter, ending next month, could be seen as "neutral or slightly positive" when taking into account the general economy.
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It added, however, that it expected to see overall Mac sales fall year-on-year.