You can't access all your files just like that in Windows Store apps. Please take a look at this MSDN page for list of locations, which your app is permited to use. Also to use most of them you will have to declare suitable Capabilities. If your file is outside this list and/or you haven't declared capabilities, you will get UnauthorizedException.
Generaly store apps shouldn't access files without user knowing about it - this is by design. If you want to access file in future, you can for example pick a file with FileOpenPicker and then access it by FutureAccessList or MostRecentlyUsed.
