Apple’s retail stores continued their year-over-year revenue slide last quarter, but a new report predicts that the brick-and-mortar locations could show growth again before the end of 2009, thanks, in part, to Windows switchers (and the iPhone 3GS of course).

In a new investment analysis from Needham & Co., Apple’s retail outlets are described as the “fourth leg of the stool.” That echoes Chief Executive Steve Jobs’ comments that his company is built on three legs: the iPod, the iPhone and the Mac. But analyst Charlie Wolf believes that Apple Stores justify the fourth leg because they play an instrumental role in attracting Windows users to the Mac platform.

Wolf highlights the stores’ minimalist design and simple shopping experience; the abundance of employees and its effect on wait times; and the plethora of experts available to help customers.

“We estimate that Windows visitors represented well over half of the 150 million visitors to the stores in fiscal 2008. Indeed, the growing volume of traffic has created its own challenges,” the report states. “With the growth of the Mac community, Apple has had to allocate increasing resources to servicing this community through the Genius and iPod bars. These post-sale services are, of course, a major reason Windows users have switched despite the perceived premium at which Macs are priced. So it’s imperative that Apple maintain the level of service it provided when the Mac installed base was much smaller.”

Wolf estimates that since 2004, Windows switchers have nearly doubled the size of the Mac install base. Of the 492,000 Macs sold in Apple Stores in the third quarter, more than half were bought by Windows users. That means that about 25 percent of Windows switchers bought their Mac at an Apple retail location.

Taking a cue from Apple, Microsoft intends to open its own retail locations this year. One of the Windows-maker’s first stores will be located near an already-established Apple Store in Mission Viejo, Calif.