You may have a policy in place restricting access to C:. Move the boot.wim to a directory.
imagex /mountrw c\somedirectory\boot.wim 1 c:\mounted_images
From Technet -
Important
You must not mount an image to the parent or the subdirectories of an already-mounted directory. Upon mounting an image to a directory that contains files, the existing files will be masked until you run the /unmount option. Additionally, you must not mount your image to Windows reserved folders.
Chris B