Apple is moving up the charts, toppling Acer to become the third largest PC vendor in the U.S., according to a survey from Gartner.

Apple defied a weakening economy to record a 38.1 percent growth rate in U.S. PC shipments, according to Gartner. Overall PC shipments in the U.S. grew just 4.2 percent to 16.5 million units during the quarter.

The company shipped 1.4 million units compared to Acer's 1.33 million, according to Gartner's survey. Dell held the top spot, shipping 5.25 million units and growing 11.9 percent year-over-year, followed by HP.

IDC also released a survey on Wednesday that had Apple and Acer running neck and neck.

Apple is witnessing unit growth across the board with both desktops and laptops doing well, said David Daoud, research manager at IDC. "They seem to have a good balance of products."

The second quarter is typically big for Apple, Daoud said. "That's when they are inking relationships and selling to educational institutions ahead of the back-to-school season," Daoud said.

In a mass market dominated by sensitive budgets and buyers, Apple is catering to specific buyers who are immune from the economic slowdown.

"The mindshare that company is significant. Not only among consumers, but small-to-medium businesses and even enterprises are looking at Apple hardware."

At the same token, budgetary constraints may make it hard for consumers to switch to high-priced Apple PCs, Daoud said.

The company is expected to sustain its growth in the U.S. in the upcoming quarters, Daoud said. Its PC shipments among students going back to school is expected to be strong in the third quarter. Apple may also find PC buyers who are unhappy with the "lack of innovation" in the PC market, Daoud said.

"[Windows] Vista may be helpful, but it is not resonating with consumers at the moment," he said.

Apple has a small market worldwide compared to HP and Dell, but it represents an opportunity to grow. The company is showing good growth in Japan, where it shipped 130,000 PC units in the first quarter of 2008, according to Daoud.