I had trouble with editing files (even editing levels) as well when I installed SMBX to C: as it was suggested by the installation program. It's better to install the game to the Program Files, there you will have less trouble.
There is a way to give yourself the rights to edit files outside of the Program Files folder (aka, in C:). You're thinking there's only one user account on your computer, but that's not true! On Windows Vista, 7 and 8, there is a hidden Guest account and a hidden user account called "Administrator"
which gives you even more rights than your user account already does;
here is a tutorial on how to enable this account. Using the "Administrator" account, I was able to give myself full access to all files on C: in the Security tab in the Properties.
BUT, do this at your own risk, since giving yourself full rights to edit all files on C: might allow other programs, wanted or unwanted, to change your files, too!