Dell (DELL) shares are sliding this morning following a pair of market research reports suggesting the company is losing market share to rival Hewlett-Packard (HPQ).

Gartner yesterday announced that HP took over the No. 1 position in worldwide PC shipments in the third quarter, the first time it has been the market leader since the fourth quarter of 2003. HP had 16.3% market share in the quarter, versus 16.1% for Dell, according to Gartner.
The firm said that while HP PC shipments increased 15.4 percent year-over-year, Dell grew 3.6%, “its lowest year-over-year growth in the company’s history.”

Overall, Gartner’s view on PC shipments was troublesome. Worldwide PC shipments totaled 59.1 million units in the third quarter of 2006, a 6.7 percent increase from the same period last year, but down 2% from the same quarter last year. It was the first year-over-year decline since the second quarter of 2002.

IDC yesterday provided a similarly discouraging picture of the PC market.

IDC says worldwide PC shipment growth slipped to 7.9% in the third quarter of the year, down from 9.8% in the second quarter and double-digit growth over the prior three years. “International shipments continued to expand at roughly 11% year on year, but slow growth in the United States pulled down overall results,” IDC said.

IDC says Dell shipments fell nearly 7% in the U.S. versus the year-ago quarter. IDC’s data shows HP and Dell in a “statistical tie” in total shipments, at 17.2%, with HP ahead by about 28,000 units.

“Dell has had some negative publicity recently regarding its battery recall but it is hard to attribute the third quarter results to bad publicity alone,” said Loren Loverde, director of IDC’s Worldwide Quarterly PC Tracker, in a statement. “Dell will likely be very aggressive in the fourth quarter, and we will see how similar battery recalls affect other vendors. These factors may cut into fourth quarter growth, but overall we don’t see a broad threat to fourth quarter growth.”

Toni Sacconaghi, hardware analyst at Bernstein Research, this morning trimmed his profit forecasts for Dell to $1.12 a share from $1.16 for this year and to $1.40 from $1.43 for next year. He says it is increasingly likely that the company will report disappointing revenues for its fiscal third quarter. Wrote Sacconaghi: “While we continue to believe that Dell has a sustained, structural advantage by virtue of its direct model, DELL’s continued struggles suggest that a turnaround may take quarters to effect, and that owning the stock at current levels will require patience.”

This morning, Dell shares are down $1.11 at $23.59. HP is up 20 cents at $39.21.