Windows includes some very complicated file permissions.
By default you will not see the full range of file permissions that you can edit. To enable this, open Windows Explorer, then Tools -> Folder Options...
A Folder Options dialog will popup.
Click on the View tab, then scroll down to the last checkbox "Use simple file sharing (Recommended)" and uncheck this box.
In Windows Explorer, right click on the file or folder you wish to change permissions for, and select "Sharing and Security...". This will now show a Security tab which was not visible before.