Convert XAP to EXE

You most likely will not find any converter that can simply convert Windows Phone Apps (.xap) to Windows executables (.exe), usually in some kind of attempt to run the file on desktop (Windows) computers. But said that, there is a Windows Phone Emulator as part of the Windows Phone SDK that can be used to launch .xap app on desktop.

Another possibility related to Silverlight packages would be to create a Windows installer for a Silverlight app, with one of the dedicated installer software like Nullsoft NSIS or similar tool with which users can create installers. A direct xap to exe conversion of any sort is not likely though.