Market research firm Canalys on Monday said it expects Apple to overtake HP and become the world’s leading PC vendor before the second half of 2012.

Canalys counts iPad as PCs and says that tablets the category has “radically changed the dynamics of the PC industry over the last year.” In fact, iPad sales have put Apple in second place in 2011 PC shipments for the third-quarter.

“Apple has seen its PC market share expand from 9% to 15% in just four quarters, though iPad shipments in its core market – the United States – are likely to come under pressure in Q4 due to the launch of the Fire and Nook at extremely competitive price points,” said Canalys Analyst Tim Coulling. “HP and Apple will fight for top position in Q4, but Apple may have to wait for the release of iPad 3 before it passes HP.”

Research from Canalys indicates PC sales should reach 415 million, up 15 percent over last year.