Assuming you have permission, and you have disabled the UAC, its likely it is still the UAC interfering. Even when it's turned off on Windows 8, its still on.

There are ways to fix this, but then it breaks the Windows 8 Modern Apps.

The only work around I have found so far is to use elevate.exe, found here:
http://jpassing.com/2007/12/08/launch-elevated-processes-from-the-command-line/

something like
shell "elevate.exe %comspec% /c copy file.ext %windir%\file.ext"