You probably don't have write permission for the top level directory. In fact, I believe by default, you shouldn't have such permission.
Try writing somewhere else instead. Since you are the admin, you can change the properties for the root directory, but it's probably not a good idea, as it was probably done to protect you from accidentally deleting the operating system.
Regardless, this is not Java related. Moving to the general computing forum.
I'm being the only Admin and User of my machine (win 7 OS)
Even admin users on Win7 have restricted rights. You can try opening a Command Prompt with the "Run as Administrator" option (right-click on Command Prompt in the Start menu to see this option). But Henry's advice is still the best - create some subdirectory under the root directory and do all of your work there.