By Aidan Malley

Published: 06:30 PM EST

An overhaul to Apple's consumer-level portables is partly credited for pushing Apple's share of the American market in May to new heights, according to a new NPD report.

The Mac maker climbed almost two full percentage points in mid-spring, securing 14.3 percent of all notebook sales in the U.S. versus 12.5 percent in April. The spike represented a 14 percent step up compared to the earlier period and lifts Apple to fourth place in the notebook sphere, putting it just behind HP, Gateway, and Toshiba.

The jump also represents a significant upward trend in the company's notebook share over the course of the spring quarter as a whole. Apple reached 9.9 percent in March, also claiming fourth place at the time. A mid-May update to the 13.3-inch MacBook is largely credited for the sharp uptick, as buyers waiting for an update were finally satisfied with new models.

And in fulfilling that need, the Cupertino-based firm also continued its trend of outrunning Windows manufacturers, NPD says. While the rival producers expanded sales by 37 percent compared to the same four-week period in 2006, MacBooks surged by 65 percent year over year -- almost double the industry rate.

Desktop share was virtually flat, however. Apple's portion of the retail business grew only slightly from 10.2 percent in April to 10.4 percent last month. The sluggish response is said to reflect an overall downturn in desktop sales which Apple has been unable to escape. The computer designer is known to be waiting on an upcoming iMac redesign for the summer.

Sales purely in the retail space were kinder to Apple's desktops, NPD says. The combined sales of iMacs, Mac minis, and Mac Pros climbed in May from 8.6 to 9.1 percent, while the more impressive notebook gain was reflected in a surge from 10.1 percent to 11.5 percent during the timeframe.

Combined share for notebooks and desktops was 13 percent, the analyst group says, marking a material increase from 11.6 percent in April.