When you talk about an App Store, right now that usually points to something Apple-related. Apple would love the trademark to that name, but Amazon's fighting that with its own "Appstore" and so is Microsoft – possibly because it will have its own application store soon too.
A leaked screenshot published on ZDNet points to what looks like a Windows application called "Windows App Store." This new feature is believed to be in the works for Windows 8, but the screenshot appears as Windows 7 in theme. If Microsoft were to launch an App Store that will be integrated with Windows 8, it's likely that it will also offer it as an add-on to the growing Windows 7 base.