Albert,
it looks like you are fighting against Windows Vista/Windows 7 UAC = User Account Control.
Typically you can't run web applications freely inside folders like Program Files, Windows directories, or system root (the C drive) folders - UAC blocks such attempts.
Try to create a new folder C:\XXXXX and install your applications (or XAMPP) there. Create moodledata folder and give users of that folder (Users of PC) all permissions using mouse right click - Properties - Security (tab) - Edit...
Sometimes you may have some Windows registry data left from old installs so finding correct localhost or 127.0.0.1 may cause headache but at least my test Vista has never had problems with parallel moodle installs (using XAMPP from C:\XAMPP).
And yes, you can change that last forward slash to backward slash...