New numbers this afternoon from the IDC research firm show worldwide PC shipments falling 3.2% during the first quarter — the first year-over-year quarterly decline since the end of the Great Recession. The firm had previously expected 1.5% growth in the worldwide market. The picture was even worse in the U.S., where shipments were down 10 percent in the recent quarter.

It’s bad news for Microsoft, which relies heavily on PC sales to fuel its Windows business.

Analysts say a variety of factors are at work, including cautious business spending, turmoil in the Middle East and Japan, and rising fuel and commodity prices. But it’s also hard to discount the huge crowds that greeted the iPad 2 launch at the beginning of March.

IDC shipment numbers for traditional PCs don’t include iPads and other “media tablets,” as they’re classified by the firm. And clearly those media tablets are capturing a portion of the consumer spending that previously went toward traditional computers.

” ‘Good-enough computing’ has become a firm reality, exemplified first by Mini Notebooks and now Media Tablets,” says Jay Chou, an IDC analyst, in a news release. “Macroeconomic forces can explain some of the ebb and flow of the PC business, but the real question PC vendors have to think hard about is how to enable a compelling user experience that can justify spending on the added horsepower.”

Here are IDC’s charts showing how individual computer vendors stacked up worldwide and in the domestic market. In the U.S., Apple pushed its way into the No. 4 position on rising Mac sales, as Acer’s shipments declined by more than 40%.

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