Needham & Company analyst Charles Wolf told clients in a note today that Apple’s retail stores have attracted lots of users from the Windows world and play a pivotal role in the Apple-Microsoft competition.

“[The Apple Stores] have become a magnet in attracting Windows users to the Mac platform,” says Wolf.

He says that — as Apple Chief Financial Officer Peter Oppenheimer has said — that Windows users purchased half of the 606,000 Macs sold in the stores during the March quarter, he added. Needham estimates that, since 2004, more than 18 million Windows users have switched to a Mac.

“Mac sales in the March quarter were almost four times higher than they were in the second quarter of fiscal 2004,” wrote Wolf. He adds that Windows switchers have more than doubled the size of the Mac installed base.

The analyst said that same-store sales rose 8% in the March quarter of 2010, thanks to a 22.3% year-over-year increase in Mac sales, even though no new Macs weren’t introduced during that time. Also, same-store non-Mac revenues at Apple’s retail locations grew 1.2% during Apple’s second fiscal quarter of 2010, thanks to strong iPad sales, he added.