"However, Apple's newest OS will be rolled out this summer and could bring more attention to the Mac platform, especially if it is rated better than Windows 7," Bajarin says.

Linux Sits Pretty on Smartphones, Netbooks

In addition to competition on client PCs, Bajarin points out a potentially more fierce battle brewing: smartphones.

The clash for OS dominance in the smartphone market puts Microsoft up against the wall, says Bajarin. Here, Microsoft is dueling with not only Apple, but Linux-based Android from Google as well as the Palm and Symbian OSs. And Google, of course, poses a much larger threat to Microsoft overall, with its dominance of the search market and ever-strengthening grip on Web-based applications that threaten Microsoft's productivity app market share.

Bajarin expects to see major vendors backing on smartphones and netbooks starting this year. "In this space, Microsoft has to be just as concerned about Android as Apple," he says.