“Canalys today announced that the PC market grew 7% in Q1 2011, as the pad market, led by Apple’s iPad, continued to bolster growth.”

That’s how Canalys, the first research firm to count tablets as PCs, began its report on how the market shaped up in the first calendar quarter of 2011.

Thanks to the iPad, Apple (AAPL) jumped into third place in worldwide PC shipments in Canalys’ Christmas quarter report, behind only HP (HPQ) and Acer. In Thursday’s report, Dell (DELL) outsold Apple by 1.5 million units, pushing Apple into fourth place. Apple might have retained its position if it could have manufactured iPad 2s fast enough to meet demand.

Total tablet shipments for the quarter reached 6.4 million units worldwide, according to Canalys, with the iPad accounting for 74% of those units.

“Taking into consideration the iPad’s ‘halo effect’ on the company’s other products, Apple has grown considerably in most markets worldwide,” said Canalys analyst Tim Coulling. “As the iPad 2 and its competitors continue to roll out, we expect pad sales to propel PC market growth for the rest of the year.”

For the rest of the Canalys report, which has a lot of detail about how people use their tablets, click here.

Also on Fortune.com:

• Counting iPads, Apple is now No. 3 in PCs
• About Apple’s P/E ratio
• Apple clobbers estimates, iPad sales fall short

[Follow Philip Elmer-DeWitt on Twitter @philiped]