Apple's share of desktop and notebook sales online and through brick-and-mortar stores rose to 13% from 11.6% in April, according to The NPD Group. Apple notebook sales rose to 14.3% of overall purchases from 12.5%, while desktop sales inched up to 10.4% from 10.2%.

"It's part of a pattern," Stephen Baker, VP of industry analysis for NPD, told InformationWeek. "They [Apple] certainly have been gaining share over the last year to year and a half."

Among the drivers in May was an upgrade of MacBook consumer notebooks to faster Intel Core 2 Duo processors, 1-Gbyte of memory, and larger hard drives in every model. "The consumer market in general is embracing notebooks, and Apple is strong in notebooks," Baker said.

In May, the overall notebook market grew by 40% over the same month a year ago. Apple outpaced overall market growth with a year-to-year jump of 65%, Baker said. Windows-based notebooks saw a 37% increase in sales. NPD does not release actual dollar amounts or unit sales.

Desktop sales in general were far less than notebooks, and neither Apple nor Windows PC vendors showed much progress. "Apple is struggling just like everybody else in terms of getting some growth out of that segment," Baker said. Apple desktop sales, however, were better than for Windows machines.

In brick and mortar stores alone, Apple notebook sales increased to 11.5% of the total from 10.1% in April. Desktop sales rose to 9.1% from 8.6%, according to NPD. Overall Mac sales in stores increased to 10.8% of the total from 9.6% in April.

Apple computer sales have been improving since its switch to the Intel platform, which also powers Windows computers, from IBM's PowerPC processor. Apple unveiled Intel-based notebooks and desktops at the 2006 Macworld Conference in San Francisco.

Also helping Mac sales is the success of the Apple iPod, which accounts for more than three quarters of the portable music player market. However, the impact of the popular device is difficult to quantify. "There's a halo affect there, but I don't know if anybody has a good idea exactly what that is," Baker said.