When you're running your service, you're not logged as the same user as that of your account. Run your service, then look at its properties:
%SystemRoot%\system32\services.msc /s
It will show Services applet; see "Log on as"; the typical users are "Local System", "Network Service", etc. Your files should be accessible by that user you used for your service. The other way is to change installation of the service to some authenticated user registered on your system.
—SA