"This is not good," Stephen Baker, NPD's vice president of industry analysis, told Macworld. "They need to address the iMac. A big chunk of this is because iMac sales are dropping two to three times compared to Windows PCs."

In its report on monthly sales trends for January, NPD blamed Apple's decline on the company's offerings aimed at bargain shoppers looking for low-cost systems. As U.S. consumers grapple with an ongoing recession, computer buyers are shifting their attention toward low-cost models. While Apple last fall and last month while maintaining its $999 price tag, the company has done little on the desktop end. The iMac line last saw an update in while the low-end Mac mini has gone a year-and-a-half without an update.

"What we're seeing is that consumers are not buying based on value, they are buying based on price. Apple is selling value," Baker said.

Interestingly, NPD's report suggests that the upper end of the computer market seems stable. People looking to buy computers priced at $1,500 and higher are less concerned with price than shoppers targeting the lower-end of the market.

In the past, Apple has been reluctant to compete for bargain-hunting computer shoppers, choosing instead to tout the quality of its product line. And that seems likely to continue, even in the current economic climate. Last October, speaking to analysts , CEO Steve Jobs said that Apple's core customers tend to delay purchases during tough times rather than seek out a cheaper brand. Jobs has also dismissed the chances of Apple building a low-cost netbook to compete with similar offerings from rival computer makers. "We don't know how to make a $500 computer that's not a piece of junk," the Apple CEO said last year.