Apple was the only top-five computer maker to post shipment gains in the U.S. during the final quarter of 2011, a pair of research firms said this week.

Both IDC and Gartner said Apple beat Acer, Dell, Hewlett-Packard (HP) and Toshiba in the U.S. last quarter in the shipment-increase game.

According to IDC estimates, Apple shipped 2 million Macs in the U.S. during the quarter ending Dec. 31, an increase of 18% over the same quarter of 2010. Gartner's number of 2.1 million was slightly higher, as was its 21% year-over-year gain by Apple.

All other computer makers in the top five shipped fewer machines in 2011 than they did the year before, IDC and Gartner said.

HP led the decline in both firms' estimates, with its shipments declining between 25.3% and 26.1%, while Acer's numbers fell between 11.4% and 14.4% from the year before.

The research companies blamed HP's waffling over whether to keep or spin off its PC business for the company's lackluster performance in the last three months of 2011, while Acer's problem stemmed from falling sales of netbooks, the inexpensive, miniature laptops that had propelled it into the top five.

Apple's estimated shipments in the fourth quarter accounted for about 11% of all computers that entered the sales channel, securing its third-place position in the U.S.

HP and Dell each shipped double the number of computers in the U.S. than did Apple.

Although neither IDC or Gartner published projections for Apple's global shipments, analysts believe Apple sold 26% to 28% more Macs worldwide last quarter than during the same period the year before.

If those estimates are accurate, Apple will set a one-quarter Mac sales record.

Apple will post its fourth-quarter earnings and sales numbers on Tuesday, Jan. 24.