Despite flat-panel iMac sales losing momentum, Apple’s U.S. and worldwide market share in the second-quarter remained little changed from a year ago. U.S. market share remained unchanged at 4.1 percent, making Apple the fifth largest computer maker, up from sixth last year, according to preliminary results from research firm IDC. Apple increased its position as a result of HP and Compaq merging, although there was no increase in overall market share: Apple shipped 453,000 units in the quarter, compared to 459,000 units a year ago. In worldwide numbers, Apple moved from eighth to seventh place — again because of the HP-Compaq merger — but actual market share dropped slightly from 2.7 percent to 2.6 percent. HP was first in both worldwide and domestic market share.