Research firm Gartner has reportedly lowered its projections for PC shipments this year and next, raising the possibility of a weaker-than-expected environment for the upcoming release of Microsoft’s Windows 8 operating system.

In a note to clients this morning, longtime Wall Street analyst Rick Sherlund of Nomura Securities said new numbers from Gartner over the weekend projected growth of 0.9 percent in PC shipments for 2012, down from its previous forecast of 4.4 percent for the year. For 2013, Gartner reduced its forecast from 11.9 percent to 11.1 percent.

The numbers don’t include tablet shipments. Microsoft is holding an event in Los Angeles today where some expect the company to unveil its own tablet computer, potentially based on some form of the Windows 8 interface. (I’m writing this on the plane to L.A. and will have more details later today from the event.)

We’ve reached out to Gartner to verify the changes in its numbers. The general trend is not good for Microsoft, which relies heavily on sales of Windows preinstalled on PCs to drive its business.

Sherlund writes that he’ll be waiting for more clarity on Microsoft’s June quarter before revisiting his own forecasts for Microsoft’s results. “Gartner has not provided an update to quarterly numbers at this stage, although we suspect that the reduced expectations are driven by uncertainty over the PC environment for the 2H’12.”

The analyst acknowledged the “controversy” over Windows 8’s new tile-based interface and full-screen apps, designed to accommodate tablets but disorienting at first for PC users accustomed to the traditional Windows interface on keyboard and mouse.

He writes, “There is a transition required and we will likely see continued mixed reactions: more negative on the legacy ‘desktop’ platform and more positive for the touch ‘Metro’ platform where all the industry growth will come from. We are still expecting RTM by late July / August, and GA by late September / early October.”

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